Gem-Stones and their Distinctive Characters

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Title: Gem-Stones and their Distinctive Characters
Author: Smith, G. F. Herbert
Language: English
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[Illustration:

_PLATE I_
_Frontispiece_

GEM-STONES]

GEM-STONES

AND THEIR DISTINCTIVE CHARACTERS

G. F. HERBERT SMITH
M.A., D.Sc.
OF THE BRITISH MUSEUM (NATURAL HISTORY)

WITH MANY DIAGRAMS AND THIRTY-TWO PLATES
OF WHICH THREE ARE IN COLOUR

THIRD EDITION

METHUEN & CO. LTD.
36 ESSEX STREET W.C.
LONDON

_First Published_ _March 21st 1910_
_Second Edition_ _June_ _1913_
_Third Edition_ _1919_

PREFACE

In this edition the opportunity has been taken to correct a few
misprints and mistakes that have been discovered in the first, and to
alter slightly one or two paragraphs, but otherwise no change has been
made. G. F. H. S. WANDSWORTH COMMON, S.W.

PREFACE TO THE FIRST EDITION

It has been my endeavour to provide in this book a concise, yet
sufficiently complete, account of the physical characters of the
mineral species which find service in jewellery, and of the methods
available for determining their principal physical constants to enable
a reader, even if previously unacquainted with the subject, to have
at hand all the information requisite for the sure identification of
any cut stone which may be met with. For several reasons I have dealt
somewhat more fully with the branches of science closely connected with
the properties of crystallized matter than has been customary hitherto
in even the most comprehensive books on precious stones. Recent years
have witnessed many changes in the jewellery world. Gem-stones are no
longer entirely drawn from a few well-marked mineral species, which
are, on the whole, easily distinguishable from one another, and it
becomes increasingly difficult for even the most experienced eye to
recognize a cut stone with unerring certainty. So long as the only
confusion lay between precious stones and paste imitations an ordinary
file was the solitary piece of apparatus required by the jeweller, but
now recourse must be had to more discriminative tests, such as the
refractive index or the specific gravity, the determination of which
calls for a little knowledge and skill. Concurrently, a keener interest
is being taken in the scientific aspect of gem-stones by the public at
large, who are attracted to them mainly by æsthetic considerations.

While the treatment has been kept as simple as possible, technical
expressions, where necessary, have not been avoided, but their meanings
have been explained, and it is hoped that their use will not prove
stumbling-blocks to the novice. Unfamiliar words of this kind often
give a forbidding air to a new subject, but they are used merely to
avoid circumlocution, and not, like the incantations of a wizard, to
veil the difficulties in still deeper gloom. For actual practical work
the pages on the refractometer and its use and the method of heavy
liquids for the determination of specific gravities, and the tables of
physical constants at the end of the book, with occasional reference,
in case of doubt, to the descriptions of the several species alone
are required; other methods—such as the prismatic mode of measuring
refractive indices, or the hydrostatic way of finding specific
gravities—which find a place in the ordinary curriculum of a physics
course are described in their special application to gem-stones, but
they are not so suitable for workshop practice. Since the scope of the
book is confined mainly to the stones as they appear on the market,
little has been said about their geological occurrence; the case of
diamond, however, is of exceptional interest and has been more fully
treated. The weights stated for the historical diamonds are those
usually published, and are probably in many instances far from correct,
but they serve to give an idea of the sizes of the stones; the English
carat is the unit used, and the numbers must be increased by about 2½
per cent. if the weights be expressed in metric carats. The prices
quoted for the various species must only be regarded as approximate,
since they may change from year to year, or even day to day, according
to the state of trade and the whim of fashion.

The diagram on Plate II and most of the crystal drawings were made
by me. The remaining drawings are the work of Mr. H. H. Penton. He
likewise prepared the coloured drawings of cut stones which appear
on the three coloured plates, his models, with two exceptions, being
selected from the cut specimens in the Mineral Collection of the
British Museum by permission of the Trustees. Unfortunately, the
difficulties that still beset the reproduction of pictures in colour
have prevented full justice being done to the faithfulness of his
brush. I highly appreciate the interest he took in the work, and the
care and skill with which it was executed. My thanks are due to the
De Beers Consolidated Mines Co. Ltd., and to Sir Henry A. Miers,
F.R.S., Principal of the University of London, for the illustrations
of the Kimberley and Wesselton diamond mines, and of the methods and
apparatus employed in breaking up and concentrating the blue ground;
to Messrs. I. J. Asscher & Co. for the use of the photograph of the
Cullinan diamond; to Mr. J. H. Steward for the loan of the block of the
refractometer; and to Mr. H. W. Atkinson for the illustration of the
diamond-sorting machine. My colleague, Mr. W. Campbell Smith, B.A., has
most kindly read the proof-sheets, and has been of great assistance in
many ways. I hope that, thanks to his invaluable help, the errors in
the book which may have escaped notice will prove few in number and
unimportant in character. To Mr. Edward Hopkins I owe an especial debt
of gratitude for his cheerful readiness to assist me in any way in
his power. He read both the manuscript and the proof-sheets, and the
information with regard to the commercial and practical side of the
subject was very largely supplied by him. He also placed at my service
a large number of photographs, some of which—for instance, those
illustrating the cutting of stones—he had specially taken for me, and
he procured for me the jewellery designs shown on Plates IV and V.

If this book be found by those engaged in the jewellery trade helpful
in their everyday work, and if it wakens in readers generally an
appreciation of the variety of beautiful minerals suitable for gems,
and an interest in the wondrous qualities of crystallized substances, I
shall be more than satisfied.
G. F. H. S.
WANDSWORTH COMMON, S.W.

CONTENTS

CHAP. PAGE
I. INTRODUCTION 1

PART I—SECTION A

THE CHARACTERS OF GEM-STONES

II. CRYSTALLINE FORM 6

III. REFLECTION, REFRACTION, AND DISPERSION 14

IV. MEASUREMENT OF REFRACTIVE INDICES 21

V. LUSTRE AND SHEEN 37

VI. DOUBLE REFRACTION 40

VII. ABSORPTION EFFECTS: COLOUR, DICHROISM,ETC. 53

VIII. SPECIFIC GRAVITY 63

IX. HARDNESS AND CLEAVABILITY 78

X. ELECTRICAL CHARACTERS 82

PART I—SECTION B

THE TECHNOLOGY OF GEM-STONES

XI. UNIT OF WEIGHT 84

XII. FASHIONING OF GEM-STONES 88

XIII. NOMENCLATURE OF PRECIOUS STONES 109

XIV. MANUFACTURED STONES 113

XV. IMITATION STONES 124

PART II—SECTION A

PRECIOUS STONES

XVI. DIAMOND 128

XVII. OCCURRENCE OF DIAMOND 137

XVIII. HISTORICAL DIAMONDS 157

XIX. CORUNDUM (_Sapphire_, _Ruby_) 172

XX. BERYL (_Emerald_, _Aquamarine_, _Morganite_) 184

PART II—SECTION B

SEMI-PRECIOUS STONES

XXI. TOPAZ 197

XXII. SPINEL (_Balas-Ruby_, _Rubicelle_) 203

XXIII. GARNET 207

(_a_) HESSONITE (_Grossular_, _Cinnamon-Stone_,
_Hyacinth_, _Jacinth_) 211

(_b_) PYROPE (‘_Cape-Ruby_’) 212

(_c_) RHODOLITE 214

(_d_) ALMANDINE (_Carbuncle_) 214

(_e_) SPESSARTITE 216

(_f_) ANDRADITE (_Demantoid_, _Topazolite_, ‘_Olivine_’) 216

(_g_) UVAROVITE 218

XXIV. TOURMALINE (_Rubellite_) 219

XXV. PERIDOT 225

XXVI. ZIRCON (_Jargoon_, _Hyacinth_, _Jacinth_) 228

XXVII. CHRYSOBERYL (_Chrysolite_, _Cat’s-Eye_, _Cymophane_,
_Alexandrite_) 233

XXVIII. QUARTZ (_Rock-Crystal_, _Amethyst_, _Citrine_,
_Cairngorm_, _Cat’s-Eye_, _Tiger’s-Eye_) 238

XXIX. CHALCEDONY, AGATE, ETC. 246

XXX. OPAL (_White Opal_, _Black Opal_, _Fire-Opal_) 249

XXXI. FELSPAR (_Moonstone_, _Sunstone_, _Labradorite_,
_Amazon-Stone_) 254

XXXII. TURQUOISE, ODONTOLITE, VARISCITE 257

XXXIII. JADE (NEPHRITE OR GREENSTONE, _Jadeite_) 260

XXXIV. SPODUMENE (_Kunzite_, _Hiddenite_), IOLITE, BENITOITE 265

XXXV. EUCLASE, PHENAKITE, BERYLLONITE 269

XXXVI. ENSTATITE (‘_Green Garnet_’), DIOPSIDE, KYANITE,
ANDALUSITE, IDOCRASE, EPIDOTE, SPHENE, AXINITE,
PREHNITE, APATITE, DIOPTASE 271

XXXVII. CASSITERITE, ANATASE, PYRITES, HEMATITE 281

XXXVIII. OBSIDIAN, MOLDAVITE 283

PART II—SECTION C

ORNAMENTAL STONES

XXXIX. FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, RHODONITE,
AZURITE, MALACHITE, THULITE, MARBLE, APOPHYLLITE,
CHRYSOCOLLA, STEATITE OR SOAPSTONE, MEERSCHAUM,
SERPENTINE 285

PART II—SECTION D

ORGANIC PRODUCTS

XL. PEARL, CORAL, AMBER 291

TABLES

I. CHEMICAL COMPOSITION OF GEM-STONES 300

II. COLOUR OF GEM-STONES 301

III. REFRACTIVE INDICES OF GEM-STONES 302

IV. COLOUR-DISPERSION OF GEM-STONES 303

V. CHARACTER OF THE REFRACTION OF GEM-STONES 303

VI. DICHROISM OF GEM-STONES 304

VII. SPECIFIC GRAVITIES OF GEM-STONES 305

VIII. DEGREES OF HARDNESS OF GEM-STONES 305

IX. DATA 306

INDEX 307

LIST OF PLATES

PAGE
I. GEM-STONES (in colour) _Frontispiece_

II. REFRACTIVE INDEX DIAGRAM 36

III. INTERFERENCE FIGURES 48

IV. JEWELLERY DESIGNS 62

V. JEWELLERY DESIGNS 88

VI. APPLIANCES USED FOR POLISHING DIAMONDS 102

VII. POLISHING DIAMONDS 103

VIII. SLITTING AND POLISHING COLOURED STONES 104

IX. FACETING MACHINE 105

X. LAPIDARY’S WORKSHOP AND OFFICE IN ENGLAND 106

XI. LAPIDARY’S WORKSHOP IN RUSSIA 107

XII. FRENCH FAMILY CUTTING STONES 108

XIII. INDIAN LAPIDARY 109

XIV. BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES
AND SAPPHIRES 118

XV. KIMBERLEY MINE, 1871 140

XVI. KIMBERLEY MINE, 1872 141

XVII. KIMBERLEY MINE, 1874 142

XVIII. KIMBERLEY MINE, 1881 143

XIX. KIMBERLEY MINE AT THE PRESENT DAY 144

XX. WESSELTON (open) MINE 145

XXI. LOADING THE BLUE GROUND ON THE FLOORS, AND PLOUGHING IT
OVER 146

XXII. WASHING-MACHINES FOR CONCENTRATING THE BLUE GROUND 147

XXIII. DIAMOND-SORTING MACHINES 148

XXIV. KAFIRS PICKING OUT DIAMONDS 149

XXV. CULLINAN DIAMOND (natural size) 168

XXVI. LARGE AQUAMARINE CRYSTAL (one-sixth natural size),
FOUND AT MARAMBAYA, MINAS GERAES, BRAZIL 196

XXVII. GEM-STONES (in colour) 226

XXVIII. OPAL MINES, WHITE CLIFFS, NEW SOUTH WALES 252

XXIX. GEM-STONES (in colour) 256

XXX. NATIVES DRILLING PEARLS 294

XXXI. METAL FIGURES OF BUDDHA INSERTED IN A PEARL-OYSTER 296

XXXII. SECTIONS OF CULTURE PEARL 297

GEM-STONES

CHAPTER I

INTRODUCTION

Beauty, durability, and rarity: such are the three cardinal virtues
of a perfect gem-stone. Stones lacking any of them cannot aspire to
a high place in the ranks of precious stones, although it does not
necessarily follow that they are of no use for ornamental purposes. The
case of pearl, which, though not properly included among gem-stones,
being directly produced by living agency, yet holds an honoured place
in jewellery, constitutes to some extent an exception, since its
incontestable beauty atones for its comparative want of durability.

That a gem-stone should be a delight to the eye is a truism that need
not be laboured; for such is its whole _raison d’être_. The members
of the Mineral Kingdom that find service in jewellery may be divided
into three groups, according as they are transparent, translucent,
or opaque. Of these the first, which is by far the largest and the
most important, may itself be further sub-divided into two sections:
stones which are devoid of colour, and stones which are tinted. Among
the former, diamond reigns supreme, since it alone possesses that
marvellous ‘fire,’ oscillating with every movement from heavenly blue
to glowing red, which is so highly esteemed and so much besought.
Other stones, such as ‘fired’ zircon, white sapphire, white topaz, and
rock-crystal, may dazzle with brilliancy of light reflected from the
surface or emitted from the interior, but none of them, like diamond,
glow with mysterious gleams. No hint of colour, save perhaps a trace
of the blue of steel, can be tolerated in stones of this category;
above all is a touch of the jaundice hue of yellow abhorred. It taxes
all the skill of the lapidary to assure that the disposition of the
facets be such as to reveal the full splendour of the stone. A coloured
stone, on the other hand, depends for its attractiveness more upon its
intrinsic hue than upon the manner of its cutting. The tint must not be
too light or too dark in shade: a stone that has barely any colour has
little interest, and one which is too dark appears almost opaque and
black. The lapidary can to some extent remedy these defects by cutting
the former deep and the latter shallow. In certain curious stones—for
instance tourmaline—the transparency, and in others—such as ruby,
sapphire, and one of the recent additions to the gem world, kunzite—the
colour, varies considerably in different directions. The colours that
are most admired—the fiery red of ruby, the royal blue of sapphire,
the verdant green of emerald, and the golden yellow of topaz—are pure
tints, and the absorption spectra corresponding to them are on the
whole continuous and often restricted. They therefore retain the purity
of their colour even in artificial light, though certain sapphires
transmit a relatively larger amount of red, and consequently turn
purple at night. Of the small group of translucent stones which pass
light, but are not clear enough to be seen through, the most important
is opal. It and certain others of the group owe their merit to the
same optical effect as that characterizing soap-bubbles, tarnished
steel, and so forth, and not to any intrinsic coloration. Another
set of stones—moonstone and the star-stones—reflect light from the
interior more or less regularly, but not in such a way as to produce
a play of colour. The last group, which comprises opaque stones, has
a single representative among ordinary gem-stones, namely, turquoise.
In this case light is scattered and reflected from layers immediately
contiguous to the surface, and the colour is due to the resulting
absorption. The apparent darkness of a deep-coloured stone follows from
a different cause: the light passing into the stone is wholly absorbed
within it, and, since none is emitted, the stone appears black. The
claims of turquoise are maintained by the blue variety; there is little
demand for stones of a greenish tinge.

It is evidently desirable that any stones used in jewellery should be
able to resist the mechanical and chemical actions of everyday life. No
one is anxious to replace jewels every few years, and the most valuable
stones are expected to endure for all time. The mechanical abrasion
is caused by the minute grains of sand that are contained in ordinary
dust, and gem-stones should be at least as hard as they—a condition
fulfilled by all the principal species with the exception of opal,
turquoise, peridot, and demantoid. Since the beauty of the first named
does not depend on the brilliancy of its polish, scratches on the
surface are not of much importance; further, all four are only slightly
softer than sand. It may be noted that the softness of paste stones,
apart from any objections that may be felt to the use of imitations,
renders them unsuitable for jewellery purposes. The only stones that
are likely to be chemically affected in the course of wear are those
which are in the slightest degree porous. It is hazardous to immerse
turquoises in liquids, even in water, lest the bluish green colour be
oxidized to the despised yellowish hue. The risk of damage to opals,
moonstones, and star-stones by the penetration of dirt or grease into
the interior of the stones is less, but is not wholly negligible.
Similar remarks apply with even greater force to pearls. Their charm,
which is due to a peculiar surface-play of light, might be destroyed by
contamination with grease, ink, or similar matter; they are, moreover,
soft. For both reasons their use in rings is much to be deprecated.
Nothing can be more unsightly than the dingy appearance of a pearl ring
after a few years’ wear.

It cannot be gainsaid that mankind prefers the rare to the beautiful,
and what is within reach of all is lightly esteemed. It is for this
reason that garnet and moonstone lie under a cloud. Purchasers can
readily be found for a ‘Cape-ruby’ or an ‘olivine,’ but not for a
garnet; garnets are so common, is the usual remark. Nevertheless,
the stones mentioned are really garnets. If science succeeded in
manufacturing diamonds at the cost of shillings instead of the pounds
that are now asked for Nature’s products—not that such a prospect is at
all probable or even feasible—we might expect them to vanish entirely
from fashionable jewellery.

A careful study of the showcases of the most extensive jewellery
establishment brings to light the fact that, despite the apparent
profusion, the number of different species represented is restricted.
Diamond, ruby, emerald, sapphire, pearl, opal, turquoise, topaz,
amethyst are all that are ordinarily asked for. Yet, as later pages
will show, there are many others worthy of consideration; two among
them—peridot and tourmaline—are, indeed, slowly becoming known. For
the first five of the stones mentioned above, the demand is relatively
steady, and varies absolutely only with the purchasing power of the
world; but a lesser known stone may suddenly spring into prominence
owing to the caprice of fashion or the preference of some great lady
or leader of fashion. Not many years ago, for instance, violet was the
favourite colour for ladies’ dresses, and consequently amethysts were
much worn to match, but with the change of fashion they speedily sank
to their former obscurity. Another stone may perhaps figure at some
royal wedding; for a brief while it becomes the vogue, and afterwards
is seldom seen.

Except that diamond, ruby, emerald, and sapphire, and, we should add,
pearl, may indisputably be considered to occupy the first rank, it is
impossible to form the gem-stones in any strict order. Every generation
sees some change. The value of a stone is after all merely what it will
fetch in the open market, and its artistic merits may be a matter of
opinion. The familiar aphorism, _de gustibus non est disputandum_, is a
warning not to enlarge upon this point.

PART I—SECTION A

THE CHARACTERS OF GEM-STONES

CHAPTER II

CRYSTALLINE FORM

With the single exception of opal, the whole of the principal mineral
species used in jewellery are distinguished from glass and similar
substances by one fundamental difference: they are crystallized matter,
and the atoms composing them are regularly arranged throughout the
structure.

The words crystal and glass are employed in science in senses differing
considerably from those in popular use. The former of them is derived
from the Greek word κρύος, meaning ice, and was at one time used in
that sense. For instance, the old fourteenth-century reading of Psalm
cxlvii. 17, which appears in the authorized version as “He giveth his
ice like morsels,” ran “He sendis his kristall as morcels.” It was
also applied to the beautiful, lustrous quartz found among the eternal
snows of the Alps, since, on account of their limpidity, these stones
were supposed, as Pliny tells us, to consist of water congealed by the
extreme cold of those regions; such at the present day is the ordinary
meaning of the word. But, when early investigators discovered that a
salt solution on evaporation left behind groups of slender glistening
prisms, each very similar to the rest, they naturally—though, as we
now know, wrongly—regarded them as representing yet another form
of congealed water, and applied the same word to such substances.
Subsequent research has shown that these salts, as well as mineral
substances occurring with natural faces in nature, have in common the
fundamental property of regularity of arrangement of the constituent
atoms, and science therefore defines by the word crystal a substance in
which the structure is uniform throughout, and all the similar atoms
composing it are arranged with regard to the structure in a similar way.

The other word is yet more familiar; it denotes the transparent,
lustrous, hard, and brittle substance produced by the fusion of sand
with soda or potash or both which fills our windows and serves a
variety of useful purposes. Research has shown that glass, though
apparently so uniform in character, has in reality no regularity of
molecular arrangement. It is, in fact, a kind of mosaic of atoms,
huddled together anyhow, but so irregular is its irregularity that
it simulates perfect regularity. Science uses the word glass in this
widened meaning. Two substances may, as a matter of fact, have the
same chemical composition, and one be a crystal and the other a glass.
For example, quartz, if heated to a high temperature, may be fused and
converted into a glass. The difference in the two types of structure
may be illustrated by a comparison between a regiment of soldiers
drawn up on parade and an ordinary crowd of people.

The crystalline form is a visible sign of the molecular arrangement,
and is intimately associated with the directional physical properties,
such as the optical characters, cleavage, etc. A study of it is not
only of interest in itself, but also of great importance to the
lapidary who wishes to cut a stone to the best advantage, and it is,
moreover, of service in distinguishing stones when in the rough state.

[Illustration: FIG. 1.—Cubo-Octahedra.]

The development of natural faces on a crystal is far from being
haphazard, but is governed by the condition that the angles between
similar faces, whether on the same crystal or on different crystals,
are equal, however varying may be the shapes and the relative sizes
of the faces (Fig. 1), and by the tendency of the faces bounding
the crystal to fall into series with parallel edges, such series
being termed zones. Each species has a characteristic type of
crystallization, which may be referred to one of the following six
systems:—

1. _Cubic._—Crystals in this system can be referred to three edges,
which are mutually at right angles, and in which the directional
characters are identical in value. These principal edges are known
as axes. Some typical forms are the cube (Fig. 2), characteristic
of fluor; the octahedron (Fig. 3), characteristic of diamond and
spinel; the dodecahedron (Fig. 4), characteristic of garnet; and the
triakisoctahedron, or three-faced octahedron (Fig. 5).

[Illustration: FIG. 2.—Cube.]

[Illustration: FIG. 3.—Octahedron.]

[Illustration: FIG. 4.—Dodecahedron.]

All crystals belonging to this system are singly refractive.

2. _Tetragonal._—Such crystals can be referred to three axes, which are
mutually at right angles, but in only two of them are the directional
characters identical. A typical form is a four-sided prism, _mm_, of
square section, terminated by four triangular faces, _p_ (Fig. 6), the
usual shape of crystals of zircon and idocrase.

[Illustration: FIG. 5.—Triakisoctahedron, or Three-faced Octahedron.]

[Illustration: FIG. 6.—Tetragonal Crystal.]

Crystals belonging to this system are doubly refractive and uniaxial,
_i.e._ they have one direction of single refraction (cf. p. 45), which
is parallel to the unequal edge of the three mentioned above.

[Illustration: FIG. 7.—Two alternative sets of Axes in the Hexagonal
System.]

3. _Hexagonal._—Such crystals can be referred alternatively either
to a set of three axes, _X_, _Y_, _Z_ (Fig. 7), which lie in a plane
perpendicular to a fourth, _H_, and are mutually inclined at angles of
60°, or to a set of three, _a_, _b_, _c_, which are not at right angles
as in the cubic system, but in which the directional characters are
identical. The fourth axis in the first arrangement is equally inclined
to each in the second set of axes. Many important species crystallize
in this system—corundum (sapphire, ruby), beryl (emerald, aquamarine),
tourmaline, quartz, and phenakite. The crystals usually display a
six-sided prism, terminated by a single face, _c_, perpendicular to the
edge of the prism _m_ (Fig. 8), _e.g._ emerald, or by six or twelve
inclined faces, _p_ (Fig. 9), _e.g._ quartz, crystals of which are so
constant in form as to be the most familiar in the Mineral Kingdom.
Tourmaline crystals (Fig. 10) are peculiar because of the fact that
often one end is obviously to the eye flatter than the other.

[Illustration: FIGS. 8-10.—Hexagonal Crystals.]

Crystals belonging to this system are also doubly refractive and
uniaxial, the direction of single refraction being parallel to the
fourth axis mentioned above, and therefore also parallel to the prism
edge. Hence deeply coloured tourmaline, which strongly absorbs the
ordinary ray, must be cut with the table-facet parallel to the edge of
the prism.

[Illustration: FIG. 11.—Relation of the two directions of single
Refraction to the Axes in an Orthorhombic Crystal.]

4. _Orthorhombic._—Such crystals can be referred to three axes, which
are mutually at right angles, but in which each of the directional
characters are different. The crystals are usually prismatic in shape,
one of the axes being parallel to the prism edge. Topaz, peridot, and
chrysoberyl are the most important species crystallizing in this system.

Crystals belonging to this system are doubly refractive and biaxial,
_i.e._ they have two directions of single refraction (cf. p. 45). The
three axes _a_, _b_, _c_ (Fig. 11) are parallel respectively to the two
bisectrices of the directions of single refraction, and the direction
perpendicular to the plane containing those directions.

5. _Monoclinic._—Such crystals can be referred to three axes, one
of which is at right angles to the other two, which are, however,
themselves not at right angles. Spodumene (kunzite) and some moonstone
crystallize in this system.

Crystals belonging to this system are doubly refractive and biaxial,
but in this case the first axis alone is parallel to one of the
principal optical directions.

6. _Triclinic._—Such crystals have no edges at right angles, and the
optical characters are not immediately related to the crystalline form.
Some moonstone crystallizes in this system.

[Illustration: FIG. 12.—Twinned Octahedron.]

Crystals are often not single separate individuals. For instance,
diamond and spinel are found in flat triangular crystals with their
girdles cleft at the corners (Fig. 12). Each of such crystals is
really composed of portions of two similar octahedra, which are placed
together in such a way that each is a reflection of the other. Such
composite crystals are called twins or macles. Sometimes the twinning
is repeated, and the individuals may be so minute as to call for a
microscope for their perception.

A composite structure may also result from the conjunction of
numberless minute individuals without any definite orientation, as in
the case of chalcedony and agate. So by supposing the individuals to
become infinitesimally small, we pass to a glass-like substance.

It is often a peculiarity of crystals of a species to display a typical
combination of natural faces. Such a combination is known as the habit
of the species, and is often of service for the purpose of identifying
stones before they are cut. Thus, a habit of diamond and spinel is
an octahedron, often twinned, of garnet a dodecahedron, of emerald a
flat-ended hexagonal prism, and so on.

It is one of the most interesting and remarkable features connected
with crystallization that the composition and the physical
characters—for instance, the refractive indices and specific
gravity—may, without any serious disturbance of the molecular
arrangement, vary considerably owing to the more or less complete
replacement of one element by another closely allied to it. That is
the cause of the range of the physical characters which has been
observed in such species as tourmaline, peridot, spinel, etc. The
principal replacements in the case of the gem-stones are ferric oxide,
Fe_{2}O_{3}, by alumina, Al_{2}O_{3}, and ferrous oxide, FeO, by
magnesia, MgO.

A list of the principal gem-stones, arranged by their chemical
composition, is given in Table I at the end of the book.

CHAPTER III

REFLECTION, REFRACTION, AND DISPERSION

It is obvious that, since a stone suitable for ornamental use must
appeal to the eye, its most important characters are those which depend
upon light; indeed, the whole art of the lapidary consists in shaping
it in such a way as to show these qualities to the best advantage. To
understand why certain forms are given to a cut stone, it is essential
for us to ascertain what becomes of the light which falls upon the
surface of the stone; further, we shall find that the action of a
stone upon light is of very great help in distinguishing the different
species of gem-stones. The phenomena displayed by light which impinges
upon the surface separating two media[1] are very similar in character,
whatever be the nature of the media.

Ordinary experience with a plane mirror tells us that, when light is
returned, or reflected, as it is usually termed, from a plane or flat
surface, there is no alteration in the size of objects viewed in this
way, but that the right and the left hands are interchanged: our right
hand becomes the left hand in our reflection in the mirror. We notice,
further, that our reflection is apparently just as far distant from the
mirror on the farther side as we are on this side. In Fig. 13 _MM´_
is a section of the mirror, and _O´_ is the image of the hand _O_ as
seen in the mirror. Light from _O_ reaches the eye _E_ by way of _m_,
but it appears to come from _O´_. Since _OO´_ is perpendicular to the
mirror, and _O_ and _O´_ lie at equal distances from it, it follows
from elementary geometry that the angle _i´_, which the reflected ray
makes with _mn_, the normal to the mirror, is equal to _i_, the angle
which the incident ray makes with the same direction.

[Illustration: FIG. 13.—Reflection at a Plane Mirror.]

Again, everyday experience tells us that the case is less simple when
light actually crosses the bounding surface and passes into the other
medium. Thus, if we look down into a bath filled with water, the
bottom of the bath appears to have been raised up, and a stick plunged
into the water seems to be bent just at the surface, except in the
particular case when it is perfectly upright. Since the stick itself
has not been bent, light evidently suffers some change in direction as
it passes into the water or emerges therefrom. The passage of light
from one medium to another was studied by Snell in the seventeenth
century, and he enunciated the following laws:—

1. The refracted ray lies in the plane containing the incident ray and
the normal to the plane surface separating the two media.

It will be noticed that the reflected ray obeys this law also.

2. The angle _r_, which the refracted ray makes with the normal, is
related to the angle _i_, which the incident ray makes with the same
direction, by the equation

_n_ sin _i_ = _n´_ sin _r_, (_a_)

where _n_ and _n´_ are constants for the two media which are known as
the indices of refraction, or the refractive indices.

This simple trigonometrical relation may be expressed in geometrical
language. Suppose we cut a plane section through the two media at right
angles to the bounding plane, which then appears as a straight line,
_SOS´_ (Fig. 14), and suppose that _IO_ represents the direction of
the incident ray; then Snell’s first law tells us that the refracted
ray _OR_ will also lie in this plane. Draw the normal _NON´_, and with
centre _O_ and any radius describe a circle intersecting the incident
and refracted rays in the points _a_ and _b_ respectively; let drop
perpendiculars _ac_ and _bd_ on to the normal _NON´_. Then we have
_n.ac = n´.bd_, whence we see that if _n_ be greater than _n´_, _ac_
is less than _bd_, and therefore when light passes from one medium
into another which is less optically dense, in its passage across the
boundary it is bent, or refracted, away from the normal.

[Illustration: FIG. 14.—Refraction across a Plane Surface.]

We see, then, that when light falls on the boundary of two different
media, some is reflected in the first and some is refracted into the
second medium. The relative amounts of light reflected and refracted
depend on the angle of incidence and the refractive indices of the
media. We shall return to this point when we come to consider the
lustre of stones.

We will proceed to consider the course of rays at different angles of
incidence when light passes out from a medium into one less dense—for
instance, from water into air. Corresponding to light with a small
angle of incidence such as _I_{1}O_ (Fig. 15), some of it is reflected
in the direction _OI´_{1}_ and the remainder is refracted out in the
direction _OR_{1}_. Similarly, for the ray _I_{2}O_ some is reflected
along _OI´_{2}_ and some refracted along _OR_{2}_. Since, in the case
we have taken, the angle of refraction is greater than the angle of
incidence, the refracted ray corresponding to some incident, ray
_I_{c}O_ will graze the bounding surface, and corresponding to a
ray beyond it, such as _I_{3}O_, which has a still greater angle of
incidence, there is no refracted ray, and all the light is wholly
or totally reflected within the dense medium. The critical angle
_I_{c}ON_, which is called the angle of total-reflection, is very
simply related to the refractive indices of the two media; for, since
_r_ is now a right angle, sin _r_ = 1, and equation (_a_) becomes

_n_ sin _i_ = _n´_ (_b_)

Hence, if the angle of total-reflection is measured and one of the
indices is known, the other can easily be calculated.

[Illustration: FIG. 15.—Total-Reflection.]

The phenomenon of total-reflection may be appreciated if we hold a
glass of water above our head, and view the light of a lamp on a table
reflected from the under surface of the water. This reflection is
incomparably more brilliant than that given by the upper surface.

The refractive index of air is taken as unity; strictly, it is that of
a vacuum, but the difference is too small to be appreciated even in
very delicate work. Every substance has different indices for light
of different colour, and it is customary to take as the standard the
yellow light of a sodium flame. This happens to be the colour to
which our eyes are most sensitive, and a flame of this kind is easily
prepared by volatilizing a little bicarbonate of soda in the flame of
a bunsen burner. A survey of Table III at the end of the book shows
clearly how valuable a measurement of the refractive index is for
determining the species to which a cut stone belongs. The values found
for different specimens of the species do in cases vary considerably
owing to the great latitude possible in the chemical constitution
due to the isomorphous replacement of one element by another. Some
variation in the index may even occur in different directions within
the same stone; it results from the remarkable property of splitting up
a beam of light into two beams, which is possessed by many crystallized
substances. This forms the subject of a later chapter.

Upon the fact that the refractive index of a substance varies for
light of different colours depends such familiar phenomena as the
splendour of the rainbow and the ‘fire’ of the diamond. When white
light is refracted into a stone it no longer remains white, but is
split up into a spectrum. Except in certain anomalous substances the
refractive index increases progressively as the wave-length of the
light decreases, and consequently a normal spectrum is violet at one
end and passes through green and yellow to red at the other end. The
width of the spectrum, which may be measured by the difference between
the refractive indices for the extreme red and violet rays, also
varies, though on the whole it increases with the refractive index. It
is the dispersion, as this difference is termed, that determines the
‘fire’—a character of the utmost importance in colourless transparent
stones, which, but for it, would be lacking in interest. Diamond excels
all colourless stones in this respect, although it is closely followed
by zircon, the colour of which has been driven off by heating; it is,
however, surpassed by two coloured species: sphene, which is seldom
seen in jewellery, and demantoid, the green garnet from the Urals,
which often passes under the misnomer ‘olivine.’ The dispersion of the
more prominent species for the _B_ and _G_ lines of the solar spectrum
is given in Table IV at the end of the book.

We will now proceed to discuss methods that may be used for the
measurement of the refractive indices of cut stones.

CHAPTER IV

MEASUREMENT OF REFRACTIVE INDICES

The methods available for the measurement of refractive indices are of
two kinds, and make use of two different principles. The first, which
is based upon the very simple relation found in the last chapter to
subsist at total-reflection, can be used with ease and celerity, and is
best suited for discriminative purposes; but it is restricted in its
application. The second, which depends on the measurement of the angle
between two facets and the minimum deviation experienced by a ray of
light when traversing a prism formed by them, is more involved, entails
the use of more elaborate apparatus, and takes considerable time, but
it is less restricted in its application.

(1) THE METHOD OF TOTAL-REFLECTION

We see from equation _b_ (p. 18), connecting the angle of
total-reflection with the refractive indices of the adjacent media,
that, if the denser medium be constant, the indices of all less dense
media may be easily determined from a measurement of the corresponding
critical angle. In all refractometers the constant medium is a glass
with a high refractive index. Some instruments have rotatory parts, by
means of which this angle is actually measured. Such instruments give
very good results, but suffer from the disadvantages of being neither
portable nor convenient to handle, and of not giving a result without
some computation.

[Illustration: FIG. 16.—Refractometer (actual size).]

For use in the identification of cut stones, a refractometer with a
fixed scale, such as that (Fig. 16) devised by the author, is far
more convenient. In order to facilitate the observations, a totally
reflecting prism has been inserted between the two lenses of the
eyepiece. The eyepiece may be adjusted to suit the individual eyesight;
but for observers with exceptionally long sight an adapter is provided,
which permits the eyepiece being drawn out to the requisite extent.
The refractometer must be held in the manner illustrated in Fig.
17, so that the light from a window or other source of illumination
enters the instrument by the lenticular opening underneath. Good,
even illumination of the field may also very simply be secured by
reflecting light into the instrument from a sheet of white paper laid
on a table. On looking down the eyepiece we see a scale (Fig. 18), the
eyepiece being, if necessary, focused until the divisions of the scale
are clearly and distinctly seen. Suppose, for experiment, we smear a
little vaseline or similar fatty substance on the plane surface of the
dense glass, which just projects beyond the level of the brass plate
embracing it. The field of view is now no longer uniformly illuminated,
but is divided into two parts (Fig. 19): a dark portion above, which
terminates in a curved edge, apparently green in colour, and a bright
portion underneath, which is composed of totally reflected light. If
we tilt the instrument downwards so that light enters the instrument
from above through the vaseline we find that the portions of the field
are reversed, the dark portion being underneath and terminated by
a red edge. It is possible so to arrange the illumination that the
two portions are evenly lighted, and the common edge becomes almost
invisible. It is therefore essential for obtaining satisfactory results
that the plate and the dense glass be shielded from the light by the
disengaged hand. The shadow-edge is curved, and is, indeed, an arc of a
circle, because spherical surfaces are used in the optical arrangements
of the refractometer; by the substitution of cylindrical surfaces it
becomes straight, but sufficient advantage is not secured thereby to
compensate for the greatly increased complexity of the construction.
The shadow-edge is coloured, because the relative dispersion,
_n{v}_ − _n{r}_
——————————————— (_n{v}_ and _n{r}_ being the refractive indices for
_n_
the extreme violet and red rays respectively), of the vaseline differs
from that of the dense glass. The dispersion of the glass is very
high, and exceeds that of any stone for which it can be used. Certain
oils have, however, nearly the same relative dispersion, and the edges
corresponding to them are consequently almost colourless. A careful eye
will perceive that the coloured shadow-edge is in reality a spectrum,
of which the violet end lies in the dark portion of the field and the
red edge merges into the bright portion. The yellow colour of a sodium
flame, which, as has already been stated, is selected as the standard
for the measurement of refractive indices, lies between the green and
the red, and the part of the spectrum to be noted is at the bottom of
the green, and practically, therefore, at the bottom of the shadow,
because the yellow and red are almost lost in the brightness of the
lower portion of the field. If a sodium flame be used as the source of
illumination, the shadow-edge becomes a sharply defined line. The scale
is so graduated and arranged that the reading where this line crosses
the scale gives the corresponding refractive index, the reading, since
the line is curved, being taken in the middle of the field on the
right-hand side of the scale. The refractometer therefore gives at
once, without any intermediate calculation, a value of the refractive
index to the second place of decimals, and a skilled observer
may, by estimating the tenths of the intervals between successive
divisions, arrive at the third place; to facilitate this estimation
the semi-divisions beyond 1·650 have been inserted. The range extends
nearly to 1·800; for any substance with a higher refractive index the
field is dark as far as the limit at the bottom.

[Illustration: FIG. 17.—Method of Using the Refractometer.]

[Illustration: FIG. 18.—Scale of the Refractometer.]

[Illustration: FIG. 19.—Shadow-edge given by a singly refractive
Substance.]

A fat, or a liquid, wets the glass, _i.e._ comes into intimate contact
with it, but if a solid substance be tested in the same way, a film of
air would intervene and entirely prevent an observation. To displace
it, a drop of some liquid which is more highly refractive than the
substance under test must first be applied to the plane surface
of the dense glass. The most convenient liquid for the purpose is
methylene iodide, CH_{2}I_{2}, which, when pure, has at ordinary room
temperatures a refractive index of 1·742. It is almost colourless when
fresh, but turns reddish brown on exposure to light. If desired, it
may be cleared in the manner described below (p. 66), but the film
of liquid actually used is so thin that this precaution is scarcely
necessary. If we test a piece of ordinary glass—one of the slips used
by microscopists for covering thin sections is very convenient for
the purpose—first applying a drop of methylene iodide to the plane
surface of the dense glass of the refractometer (Fig. 20), we notice a
coloured shadow-edge corresponding to the glass-slip at about 1·530 and
another, almost colourless, at 1·742, which corresponds to the liquid.
If the solid substance which is tested is more highly refractive than
methylene iodide, only the latter of the shadow-edges is visible, and
we must utilize some more refractive liquid. We can, however, raise
the refractive index of methylene iodide by dissolving sulphur[2] in
it; the refractive index of the saturated liquid lies well beyond
1·800 and the shadow-edge corresponding to it, therefore, does not
come within the range of the refractometer. The pure and the saturated
liquids can be procured with the instrument, the bottles containing
them being japanned on the outside to exclude light and fitted with
dipping-stoppers, by means of which a drop of the liquid required is
easily transferred to the surface of the glass of the instrument.
So long as the liquid is more highly refractive than the stone, or
whatever may be the substance under examination, its precise refractive
index is of no consequence. The facet used in the test must be flat,
and must be pressed firmly on the instrument, so that it is truly
parallel to the plane surface of the dense glass; for good results,
moreover, it must be bright.

[Illustration: FIG. 20.—Faceted Stone in Position on the Refractometer.]

[Illustration: FIG. 21.—Shadow-edges given by a doubly refractive
substance.]

We have so far assumed that the substance which we are testing is
simple and gives a single shadow-edge; but, as may be seen from Table
V, many of the gem-stones are doubly refractive, and such will,
in general, show in the field of the refractometer two distinct
shadow-edges more or less widely separated. Suppose, for example, we
study the effect produced by a peridot, which displays the phenomenon
to a marked degree. If we revolve the stone so that the facet under
observation remains parallel to the plane surface of the dense glass
of the refractometer and in contact with it, we notice that both the
shadow-edges in general move up or down the scale. In particular cases,
depending upon the relation of the position of the facet selected to
the crystalline symmetry, one or both of them may remain fixed, or
one may even move across the other. But whatever facet of the stone
be used for the test, and however variable be the movements of the
shadow-edges, the highest and lowest readings obtainable remain the
same; they are the principal indices of refraction, such as are stated
in Table III at the end of the book, and their difference measures
the maximum amount of double refraction possessed by the stone. The
procedure is therefore simplicity itself; we have merely to revolve
the stone on the instrument, usually through not more than a right
angle, and note the greatest and least readings. It will be noticed
that the shadow-edges cross the scale symmetrically in the critical and
skewwise in intermediate positions. Fig. 21 represents the effect when
the facet is such as to give simultaneously the two readings required.
The shadow-edges _a_ and _b_, which are coloured in white light,
correspond to the least and greatest respectively of the principal
refractive indices, while the third shadow-edge, which is very faint,
corresponds to the liquid used—methylene iodide. It is possible, as we
shall see in a later chapter, to learn from the motion, if any, of the
shadow-edges something as to the character of the double refraction.
Since, however, each shadow-edge is spectral in white light, they will
not be distinctly separate unless the double refraction exceeds the
relative dispersion. Topaz, for instance, appears in white light to
yield only a single shadow-edge, and may thus easily be distinguished
from tourmaline, in which the double refraction is large enough for
the separation of the two shadow-edges to be clearly discerned. In
sodium light, however, no difficulty is experienced in distinguishing
both the shadow-edges given by substances with small amount of double
refraction, such as chrysoberyl, quartz, and topaz, and a skilled
observer may detect the separation in the extreme instances of apatite,
idocrase, and beryl. The shadow-edge corresponding to the greater
refractive index is always less distinct, because it lies in the bright
portion of the field. If the stone or its facet be small, it must
be moved on the plane surface of the dense glass until the greatest
possible distinctness is imparted to the edge or edges. If it be moved
towards the observer from the further end, a misty shadow appears to
move down the scale until the correct position is reached, when the
edges spring into view.

Any facet of a stone may be utilized so long as it is flat, but the
table-facet is the most convenient, because it is usually the largest,
and it is available even when the stone is mounted. That the stone need
not be removed from its setting is one of the great advantages of this
method. The smaller the stone the more difficult it is to manipulate;
caution especially must be exercised that it be not tilted, not only
because the shadow-edge would be shifted from its true position and an
erroneous value of the refractive index obtained, but also because a
corner or edge of the stone would inevitably scratch the glass of the
instrument, which is far softer than the hard gem-stones. Methylene
iodide will in time attack and stain the glass, and must therefore be
wiped off the instrument immediately after use.

(2) THE METHOD OF MINIMUM DEVIATION

If the stone be too highly refractive for a measurement of its
refractive index to be possible with the refractometer just described,
and it is desired to determine this constant, recourse must be had
to the prismatic method, for which purpose an instrument known as
a goniometer[3] is required. Two angles must be measured; one the
interior angle included between a suitable pair of facets, and the
other the minimum amount of the deviation produced by the pair upon a
beam of light traversing them.

[Illustration: FIG. 22.—Path at Minimum Deviation of a Ray traversing a
Prism formed of two Facets of a Cut Stone.]

Fig. 22 represents a section of a step-cut stone perpendicular to a
series of facets with parallel edges; _t_ is the table, and _a, b, c_,
are facets on the culet side. The path of light traversing the prism
formed by the pair of facets, _t_ and _b_, is indicated. Suppose that
_A_ is the interior angle of the prism, _i_ the angle of incidence of
light at the first facet and _i´_ the angle of emergence at the second
facet, and _r_ and _r´_ the angles inside the stone at the two facets
respectively. Then at the first facet light has been bent through an
angle _i - r_, and again at the second facet through an angle _i´ -
r´_; the angle of deviation, _D_, is therefore given by

_D = i + i´ - (r + r´)_.

We have further that

_r + r´ = A_,

whence it follows that

_A + D = i + i´._

If the stone be mounted on the goniometer and adjusted so that the
edge of the prism is parallel to the axis of rotation of the instrument
and if light from the collimator fall upon the table-facet and the
telescope be turned to the proper position to receive the emergent
beam, a spectral image of the object-slit, or in the case of a doubly
refractive stone in general, two spectral images, will be seen in
white light; in the light of a sodium flame the images will be sharp
and distinct. Suppose that we rotate the stone in the direction of
diminishing deviation and simultaneously the telescope so as to retain
an image in the field of view, we find that the image moves up to and
then away from a certain position, at which, therefore, the deviation
is a minimum. The image moves in the same direction from this position
whichever way the stone be rotated. The question then arises what are
the angles of incidence and refraction under these special conditions.
It is clear that a path of light is reversible; that is to say, if a
beam of light traverses the prism from the facet _t_ to the facet _b_
it can take precisely the same path from the facet _b_ to the facet
_t_. Hence we should be led to expect that, since experiment teaches us
that there is only one position of minimum deviation corresponding to
the same pair of facets, the angles at the two facets must be equal,
_i.e._ _i = i´_, and _r = r´_. It is, indeed, not difficult to prove by
either geometrical or analytical methods that such is the case.

_A_ _A_ + _D_
Therefore at minimum deviation _r_ = ——— and _i_ = —————————, and,
2 2
since sin _i_ = _n_ sin _r_, where _n_ is the refractive index of
the stone, we have the simple relation—
_A_ + _D_
sin —————————
2
_n_ = —————————————
_A_
sin ———
2

This relation is strictly true only when the direction of minimum
deviation is one of crystalline symmetry in the stone, and holds
therefore in general for all singly refractive stones, and for the
ordinary ray of a uniaxial stone; but the values thus obtained even in
the case of biaxial stones are approximate enough for discriminative
purposes. If then the stone be singly refractive, the result is the
index required; if it be uniaxial, one value is the ordinary index
and the other image gives a value lying between the ordinary and the
extraordinary indices; if it be biaxial, the values given by the two
images may lie anywhere between the greatest and the least refractive
indices. The angle _A_ must not be too large; otherwise the light will
not emerge at the second facet, but will be totally reflected inside
the stone: on the other hand, it must not be too small, because any
error in its determination would then seriously affect the accuracy of
the value derived for the refractive index. Although the monochromatic
light of a sodium flame is essential for precise work, a sufficiently
approximate value for discriminative purposes is obtained by noting the
position of the yellow portion of the spectral image given in white
light.

In the case of a stone such as that depicted in Fig. 22 images are
given by other pairs of facets, for instance _ta_ and _tc_, unless the
angle included by the former is too large. There might therefore be
some doubt, to which pair some particular image corresponded; but no
confusion can arise if the following procedure be adopted.

[Illustration: FIG. 23.—Course of Observations in the Method of Minimum
Deviation.]

The table, or some easily recognizable facet, is selected as the facet
at which light enters the stone. The telescope is first placed in the
position in which it is directly opposite the collimator (_T_{0}_ in
Fig. 23), and clamped. The scale is turned until it reads exactly
zero, 0° or 360°, and clamped. The telescope is released and revolved
in the direction of increasing readings of the scale to the position
of minimum deviation, _T_. The reading of the scale gives at once the
angle of minimum deviation, _D_. The holder carrying the stone is now
clamped to the scale, and the telescope is turned to the position,
_T_{1}_,in which the image given by reflection from the table facet is
in the centre of the field of view; the reading of the scale is taken.
The telescope is clamped, and the scale is released and rotated until
it reads the angle already found for _D_. If no mistake has been made,
the reflected image from the second facet is now in the field of view.
It will probably not be quite central, as theoretically it should be,
because the stone may not have been originally quite in the position
of minimum deviation, a comparatively large rotation of the stone
producing no apparent change in the position of the refracted image at
minimum deviation, and further, because, as has already been stated,
the method is not strictly true for biaxial stones. The difference in
readings, however, should not exceed 2°. The reading, _S_, of the scale
is now taken, and it together with 180° subtracted from the reading for
the first facet, and the value of _A_, the interior angle between the
two facets, obtained.

Let us take an example.

Reading _T_ (= _D_) 40° 41´ Reading T_{1} 261° 35´
less 180° 180 0
———————
81 35
Reading _S_ 41 30
———————
½_D_ 20 20½ _A_ 40 5
½_A_ 20 2½ ½_A_ 20 2½
———————
½(_A_ + _D_) 40 23

Log sin 40° 23´ 9.81151
Log sin 20 2½ 9.53492
———————
Log _n_ 0.27659

_n_ = 1.8906.

The readings _S_ and _T_ are very nearly the same, and therefore we may
be sure that no mistake has been made in the selection of the facets.

In place of logarithm-tables we may make use of the diagram on Plate
II. The radial lines correspond to the angles of minimum deviation and
the skew lines to the prism angles, and the distance along the radial
lines gives the refractive index. We run our eye along the line for the
observed angle of minimum deviation and note where it meets the curve
for the observed prism angle; the refractive index corresponding to the
point of intersection is at once read off.

This method has several obvious disadvantages: it requires the use
of an expensive and elaborate instrument, an observation takes
considerable time, and the values of the principal refractive indices
cannot in general be immediately determined.

Table III at the end of the book gives the refractive indices of the
gem-stones.

[Illustration: _PLATE II_

REFRACTIVE INDEX DIAGRAM]

CHAPTER V

LUSTRE AND SHEEN

It has been already stated that whenever light in one medium falls
upon the surface separating it from another medium some of the light
is reflected within the first, while the remainder passes out into
the second medium, except when the first is of lower refractivity
than the second and light falls at an angle greater than that of
total-reflection. Similarly, when light impinges upon a cut stone
some of it is reflected and the remainder passes into the stone. What
is the relative amount of reflected light depends upon the nature of
the stone—its refractivity and hardness—and determines its lustre;
the greater the amount the more lustrous will the stone appear. There
are different kinds of lustre, and the intensity of each depends on
the polish of the surface. From a dull, _i.e._ an uneven, surface the
reflected light is scattered, and there are no brilliant reflections.
All gem-stones take a good polish, and have therefore, so long as the
surface retains its polish, considerable brilliancy; turquoise, on
account of its softness, is always comparatively dull.

The different kinds of lustre are—

(1) Adamantine, characteristic of diamond.
(2) Vitreous, as seen on the surface of fractured glass.
(3) Resinous, as shown by resins.

Zircon and demantoid, the green garnet called by jewellers “olivine,”
alone among gem-stones have a lustre approaching that of diamond. The
remainder all have a vitreous lustre, though varying in degree, the
harder and the more refractive species being on the whole the more
lustrous.

Some stones—for instance, a cinnamon garnet—appear to have a certain
greasiness in the lustre, which is caused by stray reflections from
inclusions or other breaks in the homogeneity of the interior. A pearly
lustre, which arises from cleavage cracks and is typically displayed
by the cleavage face of topaz, would be seen in a cut stone only when
flawed.

Certain corundums when viewed in the direction of the
crystallographical axis display six narrow lines of light radiating at
angles of 60° from a centre in a manner suggestive of the conventional
representations of stars. Such stones are consequently known as
asterias, or more usually star-stones—star-rubies or star-sapphires, as
the case may be, and the phenomenon is called asterism. These stones
have not a homogeneous structure, but contain tube-like cavities
regularly arranged at angles of 60° in planes at right angles to the
crystallographical axis. The effect is best produced when the stones
are cut _en cabochon_ perpendicular to that axis.

Chatoyancy is a somewhat similar phenomenon, but in this case the
fibres or cavities are parallel to a single direction, and a single
broadish band is displayed at right angles to it. Cat’s-eyes, as these
stones are termed, are cut _en cabochon_ parallel to the fibres. The
true cat’s-eye (Plate XXIX, Fig. 1) is a variety of chrysoberyl, but
the term is also often applied to quartz showing a similar appearance.
The latter is really a fibrous mineral, such as asbestos, which has
become converted into silica. The beautiful tiger’s-eye from South
Africa is a silicified crocidolite, the original blue colour of which
has been altered by oxidation to golden brown. Recently tourmalines
have been discovered which are sufficiently fibrous in structure to
display an effective chatoyancy.

The milky sheen of moonstone (Plate XXIX, Fig. 4) owes its effect to
reflections from twin lamellæ. The wonderful iridescence which is
the glory of opal, and is therefore termed opalescence, arises from
a structure which is peculiar to that species. Opal is a solidified
jelly; on cooling it has become riddled with extremely thin cracks,
which were subsequently filled with similar material of slightly
different refractivity, and thus it consists of a series of films. At
the surface of each film interference of light takes place just as
at the surface of a soap-bubble, and the more evenly the films are
spaced apart the more uniform is the colour displayed, the actual tint
depending upon the thickness of the films traversed by the light giving
rise to the phenomenon.

CHAPTER VI

DOUBLE REFRACTION

The optical phenomenon presented by many gem-stones is complicated
by their property of splitting up a beam of light into two with, in
general, differing characters. In this chapter we shall discuss the
nature of double refraction, as it is termed, and methods for its
detection. The phenomenon is not one that comes within the purview of
everyday experience.

So long ago as 1669 a Danish physician, by name Bartholinus, noticed
that a plate of the transparent mineral which at that time had
recently been brought over from Iceland, and was therefore called
“Iceland-spar,” possessed the remarkable property of giving a double
image of objects close to it when viewed through it. Subsequent
investigation has shown that much crystallized matter is doubly
refractive, but in calcite—to use the scientific name for the species
which includes Iceland-spar—alone among common minerals is the
phenomenon so conspicuous as to be obvious to the unaided eye. The
apparent separation of the pair of images given by a plate cut or
cleaved in any direction depends upon its thickness. The large mass,
upwards of two feet (60 cm.) in thickness, which is exhibited at
the far end of the Mineral Gallery of the British Museum (Natural
History), displays the separation to a degree that is probably unique.

[Illustration: FIG. 24.—Apparent doubling of the Edges of a Peridot
when viewed through the Table-Facet.]

Although none of the gem-stones can emulate calcite in this character,
yet the double refraction of certain of them is large enough to be
detected without much difficulty. In the case of faceted stones the
opposite edges should be viewed through the table-facet, and any signs
of doubling noted. The double refraction of sphene is so large, viz.
0·08, that the doubling of the edges is evident to the unaided eye.
In peridot (Fig. 24), zircon (b), and epidote the apparent separation
of the edges is easily discerned with the assistance of an ordinary
lens. A keen eye can detect the phenomenon even in the case of such
substances as quartz with small double refraction. It must, however, be
remembered that in all such stones the refraction is single in certain
directions, and the amount of double refraction varies therefore
with the direction from nil to the maximum possessed by the stone.
Experiment with a plate of Iceland-spar shows that the rays transmitted
by it have properties differing from those of ordinary light. On
superposing a second plate we notice that there are now two pairs of
images, which are in general no longer of equal brightness, as was the
case before. If the second plate be rotated with respect to the first,
two images, one of each pair, disappear, and then the other two, the
plate having turned through a right angle between the two positions of
extinction; midway between these positions the images are all equally
bright. This variation of intensity implies that each of the rays
emerging from the first plate has acquired a one-sided character, or,
as it is usually expressed, has become plane-polarized, or, shortly,
polarized.

[Illustration: FIG. 25.—Wave-Motion.]

Before the discovery of the phenomenon of double refraction the
foundation of the modern theory of light had been laid by the genius of
Huygens. According to this theory light is the result of a wave-motion
(Fig. 25) in the ether, a medium that pervades the whole of space
whether occupied by matter or not, and transmits the wave-motion at a
rate varying with the matter with which it happens to coincide. Such
a medium has been assumed because it explains satisfactorily all the
phenomena of light, but it by no means follows that it has a concrete
existence. Indeed, if it has, it is so tenuous as to be imperceptible
to the most delicate experiments. The wave-motion is similar to that
observed on the surface of still water when disturbed by a stone flung
into it. The waves spread out from the source of disturbance; but,
although the waves seem to advance, the actual particles of water
merely move up and down, and have no motion at all in the direction
in which the waves are moving. If we imagine similar motion to take
place in any plane and not only the horizontal, we form some idea
of the nature of ordinary light. But after passing through a plate
of Iceland-spar, light no longer vibrates in all directions, but in
each beam the vibrations are parallel to a particular plane, the two
planes being at right angles. The exact relation of the direction of
the vibrations to the plane of polarization is uncertain, although it
undoubtedly lies in the plane containing the direction of the ray of
light and the perpendicular to the plane of polarization. The waves for
different colours differ in their length, _i.e._ in the distance, 2
_bb_ (Fig. 25), from crest to crest, while the velocity, which remains
the same for the same medium, is proportional to the wave-length. The
intensity of the light varies as the square of the amplitude of the
wave, _i.e._ the height, _ab_, of the crest from the mean level.

Various methods have been proposed for obtaining polarized light. Thus
Seebeck found in 1813 that a plate of brown tourmaline cut parallel
to the crystallographic axis and of sufficient thickness (cf. p. 11)
transmits only one ray, the other being entirely absorbed within the
plate. Another method was to employ a glass plate to reflect light at a
certain critical angle. The most efficient method, and that in general
use at the present day, is due to the invention of Nicol. A rhomb of
Iceland-spar (Fig. 26), of suitable length, is sliced along the longer
diagonal, _dd_, and the halves are cemented together by means of canada
balsam. One ray, _ioo_, is totally reflected at the surface separating
the mineral and the cement, and does not penetrate into the other half;
while the other ray, _iee_, is transmitted with almost undiminished
intensity. Such a rhomb is called a Nicol’s prism after its inventor,
or briefly, a nicol.

[Illustration: FIG. 26.—Nicol’s Prism.]

If one nicol be placed above another and their corresponding principal
planes be at right angles no light is transmitted through the pair.
In the polarizing microscope one such nicol, called the polarizer, is
placed below the stage, and the other, called the analyser, is either
inserted in the body of the microscope or placed above the eyepiece,
and the pair are usually set in the crossed position so that the field
of the microscope is dark. If a piece of glass or a fragment of some
singly refractive substance be placed on the stage the field still
remains dark; but in case of a doubly refractive stone the field is
no longer dark except in certain positions of the stone. On rotation
of the plate, or, if possible, of the nicols together, the field
passes from darkness to maximum brightness four times in a complete
revolution, the relative angular intervals between these positions
being right angles. These positions of darkness are known as the
positions of extinction, and the plate is said to extinguish in them.
This test is exceedingly delicate and reveals the double refraction
even when the greatest difference in the refractive indices is too
small to be measured directly.

Doubly refractive substances are of two kinds: uniaxial, in which
there is one direction of single refraction, and biaxial, in which
there are two such directions. In the case of the former the direction
of one, the ordinary ray, is precisely the same as if the refraction
were single, but the refractive index of the other ray varies from
that of the ordinary ray to a second limiting value, the extraordinary
refractive index, which may be either greater or less. If the
extraordinary is greater than the ordinary refractive index the double
refraction is said to be positive; if less, to be negative. A biaxial
substance is more complex. It possesses three principal directions,
viz., the bisectrices of the directions of single refraction and the
perpendicular to the plane containing them. The first two correspond
to the greatest and least, and the last to the mean of the principal
indices of refraction. If the acute bisectrix corresponds to the
least refractive index, the double refraction is said to be positive,
and if to the greatest, negative. The relation of the directions of
single refraction, _s_, to the three principal directions, _a, b, c_,
is illustrated in Fig. 27 for the case of topaz, a positive mineral.
The refractive indices of the rays traversing one of the principal
directions have the values corresponding to the other two. In the
direction _a_ we should measure the greatest and the mean of the
principal refractive indices, in the direction _b_ the greatest and the
least, and in the direction _c_ the mean and the least. The maximum
amount of double refraction is therefore in the direction _b_.

[Illustration: FIG. 27.—Relation of the two Directions of single
Refraction to the principal Optical Directions in a Biaxial Crystal.]

In the examination of a faceted stone, of the most usual shape, the
simplest method is to lay the large facet, called the table, on a glass
slip and view the stone through the small parallel facet, the culet.
Should the latter not exist, it may frequently happen that owing to
internal reflection no light emerges through the steeply inclined
facets. This difficulty is easily overcome by immersing the stone in
some highly refracting oil. A glass plate held by hand over the stone
with a drop of the oil between it and the plate serves the purpose, and
is perhaps a more convenient method. A stone which does not possess a
pair of parallel facets should be viewed through any pair which are
nearly parallel.

We have stated that a plate of glass has no effect on the field.
Suppose, however, it were viewed when placed between the jaws of a
tightened vice and thus thrown into a state of strain, it would then
show double refraction, the amount of which would depend on the strain.
Natural singly refractive substances frequently show phenomena of a
similar kind. Thus diamond sometimes contains a drop of liquid carbonic
acid, and the strain is revealed by the coloured rings surrounding
the cavity which are seen when the stone is viewed between crossed
nicols. Double refraction is also common in diamond even when there is
no included matter to explain it, and is caused by the state of strain
into which the mineral is thrown on release from the enormous pressure
under which it was formed. Other minerals which display these so-called
optical anomalies, such as fluor and garnet, are not really quite
singly refractive at ordinary temperatures; each crystal is composed
of several double refractive individuals. But all such phenomena
cannot be confused with the characters of minerals which extinguish
in the ordinary way, since the stone will extinguish in small patches
and these will not be dark all at the same time; further, the double
refraction is small, and on revolving the stone between crossed nicols
the extinction is not sharp. Paste stones are sometimes in a state of
strain, and display slight, but general, double refraction. Hence the
existence of double refraction does not necessarily prove that the
stone is real and not an imitation. Stones may be composed of two or
more individuals which are related to each other by twinning, in which
case each individual would in general extinguish separately. Such
individuals would be larger and would extinguish more sharply than the
patches of an anomalous stone.

[Illustration: FIG. 28.—Interference of Light.]

An examination in convergent light is sometimes of service. An
auxiliary lens is placed over the condenser so as to converge the light
on to the stone. Light now traverses the stone in different directions;
the more oblique the direction the greater the distance traversed in
the stone. If it be doubly refractive, in any given direction there
will be in general two rays with differing refractive indices and the
resulting effect is akin to the well-known phenomenon of Newton’s
rings, and is an instance of what is termed interference. It may be
mentioned that the interference of light (Fig. 28) explains such common
phenomena as the colours of a soap-bubble, the hues of tarnished steel,
the tints of a layer of oil floating on water, and so on. Light, after
diverging from the stone, comes to focus a little beneath the plane
in which the image of the stone is formed. An auxiliary lens must,
therefore, be inserted to bring the focal planes together, so that the
interference picture may be viewed by means of the same eyepiece.

If a uniaxial crystal be examined along the crystallographic axis in
convergent light an interference picture will be seen of the kind
illustrated on Plate III. The arms of a black cross meet in the centre
of the field, which is surrounded by a series of circular rings,
coloured in white light. Rotation of the stone about the axis
produces no change in the picture.

[Illustration: _PLATE III_

1. UNIAXIAL

2. UNIAXIAL (_Circular Polarization_)

3. BIAXIAL (_Crossed Brushes_)

4. BIAXIAL (_Hyperbolic Brushes_)

INTERFERENCE FIGURES]

A biaxial substance possesses two directions (_the optic axes_) along
which a single beam is transmitted. If such a stone be examined along
the line bisecting the acute angle between the optic axes (_the acute
bisectrix_) an interference picture[4] will be seen which in particular
positions of the stone with respect to the crossed nicols takes the
forms illustrated on Plate III. As before, there is a series of rings
which are coloured in white light; they, however, are no longer circles
but consist of curves known as lemniscates, of which the figure of 8
is a special form. Instead of an unchangeable cross there are a pair
of black “brushes” which in one position of the stone are hyperbolæ,
and in that at right angles become a cross. On rotating the stone we
find that the rings move with it and are unaltered in form, whereas the
brushes revolve about two points, called the “eyes,” where the optic
axes emerge. If the observation were made along the obtuse bisectrix
the angle between the optic axes would probably be too large for the
brushes to come into the field, and the rings might not be visible in
white light, though they would appear in monochromatic light. In the
case of a substance like sphene the figure is not so simple, because
the positions of the optic axes vary greatly for the different colours
and the result is exceedingly complex; in monochromatic light, however,
the usual figure is visible.

It would probably not be possible in the case of a faceted stone
to find a pair of faces perpendicular to the required direction.
Nevertheless, so long as a portion of the figures described is in the
field of view, the character of the double refraction, whether uniaxial
or biaxial, may readily be determined.

There is yet another remarkable phenomenon which must not be passed
over. Certain substances, of which quartz is a conspicuous example and
in this respect unique among the gem-stones, possess the remarkable
property of rotating the plane of polarization of a ray of light which
is transmitted parallel to the optic axis. If a plate of quartz be
cut at right angles to the axis and placed between crossed nicols in
white light, the field will be coloured, the hue changing on rotation
of one nicol with respect to the other. Examination in monochromatic
light shows that the field will become dark after a certain rotation of
the one nicol with respect to the other, the amount of which depends
on the thickness of the plate. If the plate be viewed in convergent
light, an interference picture is seen as illustrated on Plate III,
which is similar to, and yet differs in some important particulars
from the ordinary interference picture of a uniaxial stone. The cross
does not penetrate beyond the innermost ring and the centre of the
field is coloured in white light. If a stone shows such a picture, it
may be safely assumed to be quartz. It is interesting to note that
minerals which possess this property have a spiral arrangement of the
constituent atoms.

It has already been remarked (p. 28) that if a faceted doubly
refractive stone be rotated with one facet always in contact with the
dense glass of the refractometer the pair of shadow-edges that are
visible in the field move up or down the scale in general from or
to maximum and minimum positions. The manner in which this movement
takes place depends upon the character of the double refraction and
the position of the facet under observation with regard to the optical
symmetry of the stone. In the case of a uniaxial stone, if the facet
be perpendicular to the crystallographic axis, i.e. the direction
of single refraction, neither of the shadow-edges will move. If the
facet be parallel to that direction, one shadow-edge will move up and
coincide with the other, which remains invariable in position, and
away from it to a second critical position; the latter gives the value
of the extraordinary refractive index, and the invariable shadow-edge
corresponds to the ordinary refractive index. This phenomenon is
displayed by the table-facet of most tourmalines, because for
reasons given above (p. 11) they are as a rule cut parallel to the
crystallographic axis. In the case of facets in intermediate positions,
the shadow-edge corresponding to the extraordinary refractive index
moves, but not to coincidence with the invariable shadow-edge. The case
of a biaxial stone is more complex. If the facet be perpendicular to
one of the principal directions one shadow-edge remains invariable in
position, corresponding to one of the principal refractive indices,
whilst the other moves between the critical values corresponding
to the remaining two of the principal refractive indices. In the
interesting case in which the facet is parallel to the two directions
of single refraction, the second shadow-edge moves across the one which
is invariable in position. In intermediate positions of the facet
both shadow-edges move, and give therefore critical values. Of the
intermediate pair, _i.e._ the lower maximum and the higher minimum,
one corresponds to the mean principal refractive index, and the other
depends upon the relation of the facet to the optical symmetry. If it
is desired to distinguish between them, observations must be made on
a second facet; but for discriminative purposes such exactitude is
unnecessary, since the least and the greatest refractive indices are
all that are required.

The character of the refraction of gem-stones is given in Table V at
the end of the book.

CHAPTER VII

ABSORPTION EFFECTS: COLOUR, DICHROISM, ETC.

When white light passes through a cut stone, colour effects result
which arise from a variety of causes. The most obvious is the
fundamental colour of the stone, which is due to its selective
absorption of the light passing through it, and would characterize
it before it was cut. Intermingled with the colour in a transparent
stone is the dispersive effect known as ‘fire,’ which has already
been discussed (p. 20). In many instances the want of homogeneity is
responsible for some peculiar effects such as opalescence, chatoyancy,
and asterism. These phenomena will now be considered in fuller detail.

COLOUR

All substances absorb light to some extent. If the action is slight and
affects equally the whole of the visible spectrum, the stone appears
white or colourless. Usually some portion is more strongly absorbed
than the rest, and the stone seems to be coloured. What is the precise
tint depends not only upon the portions transmitted through the stone,
but also upon their relative intensities. The eye, unlike the ear, has
not the power of analysis and it cannot of itself determine how a
composite colour has been made up. Indeed, so far as it is concerned,
any colour may be exactly matched by compounding in certain proportions
three simple primary colours—red, yellow, and violet. Alexandrite, a
variety of chrysoberyl, is a curious and instructive case. The balance
in the spectrum of light transmitted through it is such that, whereas
in daylight such stones appear green, in artificial light, especially
in gas-light, they are a pronounced raspberry-red (Plate XXVII,
Figs. 11, 13). The phenomenon is intensified by the strong dichroism
characteristic of this species.

The colour is the least reliable character that may be employed for the
identification of a stone, since it varies considerably in the same
species, and often results from the admixture of some metallic oxide,
which has no essential part in the chemical composition and is present
in such minute quantities as to be almost imperceptible by analysis.
Who would, for instance, imagine from their appearance that stones so
markedly diverse in hue as ruby and sapphire were really varieties of
the same species, corundum? Again, quartz, in spite of the simplicity
of its composition, displays extreme differences of tint. Nevertheless,
certain varieties do possess a distinctive colour, emerald being
the most striking example, and in other cases the trained eye can
appreciate certain characteristic subtleties of shade. At any rate, the
colour is the most obvious of the physical characters, and serves to
provide a rough division of the species, and accordingly in Table II at
the end of the book the gem-stones are arranged by their usual tints.

DICHROISM

The two rays into which a doubly refractive stone splits up a ray of
light are often differently absorbed by it, and in consequence appear
on emergence differently coloured; such stones are said to be dichroic.
The most striking instance is a deep-brown tourmaline, which, except
in very thin sections, is quite opaque to the ordinary ray. The light
transmitted by a plate cut parallel to the crystallographic axis is
therefore plane-polarized; before the invention by Nicol of the prism
of Iceland-spar known by his name this was the ordinary method of
obtaining light of this character (cf. p. 43). Again, in the case of
kunzite and cordierite the difference in colour is so marked as to be
obvious to the unaided eye; but where the contrast is less pronounced
we require the use of an instrument called a dichroscope, which enables
the twin colours to be seen side by side.

[Illustration: FIG. 29.—Dichroscope (actual size).]

[Illustration: FIG. 30.—Field of the Dichroscope.]

Fig. 29 illustrates in section the construction of a dichroscope. The
instrument consists essentially of a rhomb of Iceland-spar, _S_, of
such a length as to give two contiguous images (Fig. 30) of a square
hole, _H_, in one end of the tube containing it. In some instruments
the terminal faces of the rhomb are ground at right angles to its
length, but usually, as in that depicted, prisms of glass, _G_, are
cemented on to the two ends. A cap _C_, with a slightly larger hole,
which is circular in shape, fits on the end of the tube, and can be
moved up and down it and revolved round it, as desired. The stone, _R_,
to be tested may be directly attached to it by means of some kind of
wax or cement in such a way that light which has traversed it passes
into the window, _H_, of the instrument; the cap at the same time
permits of the rotation of the stone about the axis of the main tube of
the instrument. The dichroscope shown in the figure has a still more
convenient arrangement: it is provided with an additional attachment,
_A_, by means of which the stone can be turned about an axis at right
angles to the length of the tube, and thus examined in different
directions. At the other end of the main tube is placed a lens, _L_, of
low power for viewing the twin images: the short tube containing it can
be pushed in and out for focusing purposes. Many makers now place the
rhomb close to the lens, _L_, and thereby require a much smaller piece
of spar; material suitable for optical purposes is fast growing scarce.

Suppose that a plate of tourmaline cut parallel to its crystallographic
axis is fastened to the cap and the latter rotated. We should notice,
on looking through the instrument, that in the course of a complete
revolution there are two positions, orientated at right angles to
one another, in which the tints of the two images are identical, the
positions of greatest contrast of tint being midway between. If we
examine a uniaxial stone in a direction at right angles to its optic
axis we obtain the colours corresponding to the ordinary and the
extraordinary rays. In any direction less inclined to the axis we
still have the colour for the ordinary ray, but the other colour is
intermediate in tint between it and that for the extraordinary ray.
The phenomenon presented by a biaxial stone is more complex. There
are three principal colours which are visible in differing pairs in
the three principal optical directions; in other directions the tints
seen are intermediate between the principal colours. Since biaxial
stones have three principal colours, they are sometimes said to be
trichroic or pleochroic, but in any single direction they have two
twin colours and show dichroism. No difference at all will be shown in
directions in which a stone is singly refractive, and it is therefore
always advisable to examine a stone in more than one direction lest
the first happens to be one of single refraction. For determinative
purposes it is not necessary to note the exact shades of tint of the
twin colours, because they vary with the inherent colour of the stone,
and are therefore not constant for the same species; we need only
observe, when the stone is tested with the dichroscope, whether there
is any variation of colour, and, if so, its strength. Dichroism is a
result of double refraction, and cannot exist in a singly refractive
stone. The converse, however, is not true and it by no means follows
that, because no dichroism can be detected in a stone, it is singly
refractive. A colourless stone, for instance, cannot possibly be
dichroic, and many coloured, doubly refractive stones—for example,
zircon—exhibit no dichroism, or so little that it is imperceptible.
The character is always the better displayed, the deeper the inherent
colour of the stone. The deep-green alexandrite, for instance, is far
more dichroic than the lighter coloured varieties of chrysoberyl.

If the stone is attached to the cap of the instrument, the table should
be turned towards it so as to assure that the light passing into the
instrument has actually traversed the stone. If little light enters
through the opposite coign, a drop of oil placed thereon will overcome
the difficulty (cf. p. 46). It is also necessary, for reasons mentioned
above, to examine the stone in directions as far as possible across
the girdle also. A convenient, though not strictly accurate, method
is to lay the stone with the table facet on a table and examine the
light which has entered the stone and been reflected at that facet. The
stone may easily be rotated on the table, and observations thus made
in different directions in the stone. Care must be exercised in the
case of a faceted stone not to mistake the alteration in colour due to
dispersion for a dichroic effect, and the stone must be placed close to
the instrument during an observation, because otherwise the twin rays
traversing the instrument may have taken sensibly different directions
in the stone.

Dichroism is an effective test in the case of ruby; its twin
colours—purplish and yellowish red—are in marked contrast, and readily
distinguish it from other red stones. Again, one of the twin colours
of sapphire is distinctly more yellowish than the other; the blue
spinel, of which a good many have been manufactured during recent
years, is singly refractive, and, of course, shows no difference of
tint in the dichroscope.

Table VI at the end of the book gives the strength of the dichroism of
the gem-stones.

ABSORPTION SPECTRA

A study of the chromatic character of the light transmitted by a
coloured stone is of no little interest. As was stated above, the eye
has not the power of analysing light, and to resolve the transmitted
rays into their component parts an instrument known as a spectroscope
is needed. The small ‘direct-vision’ type has ample dispersion for this
purpose. It is advantageous to employ by preference the diffraction
rather than the prism form, because in the former the intervals in the
resulting spectrum corresponding to equal differences of wave-length
are the same, whereas in the latter they diminish as the wave-length
increases and accordingly the red end of the spectrum is relatively
cramped.

The absorptive properties of all doubly refractive coloured substances
vary more or less with the direction in which light traverses them
according to the amount of dichroism that they possess, but the
variation is not very noticeable unless the stone is highly dichroic.
If the light transmitted by a deep-coloured ruby be examined with a
spectroscope it will be found that the whole of the green portion of
the spectrum is obliterated (Fig. 31), while in the case of a sapphire
only a small portion of the red end of the spectrum is absorbed.
Alexandrite affords especial interest. In the spectrum of the light
transmitted by it, the violet and the yellow are more or less strongly
absorbed, depending upon the direction in which the rays have passed
through the stone (Fig. 31), and the transmitted light is mainly
composed of two portions—red and green. The apparent colour of the
stone depends, therefore, upon which of the two predominates. In
daylight the resultant colour is green flecked with red and orange,
the three principal absorptive tints (cf. p. 235), but in artificial
light, which is relatively stronger in the red portion of the spectrum,
the resultant colour is a raspberry-red, and there is less apparent
difference in the absorptive tints (cf. Plate XXVII, Figs. 11, 13).

[Illustration: FIG. 31.—Absorption Spectra.]

In all the spectra just considered, and in all like them, the portions
that are absorbed are wide, the passage from blackness to colour is
gradual, and the edges deliminating them are blurred. In the spectra of
certain zircons and in almandine garnet the absorbed portions, or bands
as they are called, are narrow, and, moreover, the transition from
blackness to colour is sharp and abrupt; such stones are therefore said
to display absorption-bands. Church in 1866 was the first to notice
the bands shown by zircon (Fig. 31). Sorby thought they portended the
existence of a new element, to which he gave the name jargonium, but
subsequently discovered that they were caused by the presence of a
minute trace of uranium. A yellowish-green zircon shows the phenomenon
best, and it has all the bands shown in the figure. The spectrum varies
slightly but almost imperceptibly with the direction in the stone.
Others show the bands in the yellow and green, while others show only
those in the red, and some only one of them. The bands are not confined
to stones of any particular colour, or amount of double refraction.
Again, many zircons show no bands at all, so that their absence by no
means precludes the stone from being a zircon.

Almandine is characterized by a different spectrum (Fig. 31). The band
in the yellow is the most conspicuous, and is no doubt responsible for
the purple hue of a typical almandine. The spectrum varies in strength
in different stones. Rhodolite (p. 214), a garnet lying between
almandine and pyrope, displays the same bands, and indications of them
may be detected in the spectra of pyropes of high refraction.

[Illustration: _PLATE IV_

JEWELLERY DESIGNS]

CHAPTER VIII

SPECIFIC GRAVITY

It is one of our earliest experiences that different substances of
the same size have often markedly different weights; thus, there is
a great difference between wood and iron, and still greater between
wood and lead. It is usual to say that iron is heavier than wood,
but the statement is misleading, because it would be possible by
selecting a large enough piece of wood to find one at least as heavy
as a particular piece of iron. We have, in fact, to compare equal
volumes of the two substances, and all ambiguity is removed if we
speak of relative density or specific gravity—the former term being
usually applied to liquids and the latter to solids—instead of weight
or heaviness. The density of water at 4° C. is taken as unity, that
being the temperature at which it is highest; at other temperatures it
is somewhat lower, as will be seen from Table IX given at the end of
the book. The direct determination of the volume of an irregular solid
presents almost insuperable difficulty; but, fortunately, for finding
the specific gravity it is quite unnecessary to know the volume, as
will be shown when we proceed to consider the methods in use.

The specific gravity of a stone is a character which is within narrow
limits constant for each species, and is therefore very useful for
discriminative purposes. It can be determined whatever be the shape
of the stone, and it is immaterial whether it be transparent or not;
but, on the other hand, the stone must be unmounted and free from the
setting.

The methods for the determination of the specific gravity are of two
kinds: in the first a liquid is found of the same, or nearly the same,
density as the stone, and in the second weighings are made and the use
of an accurate balance is required.

(1) HEAVY LIQUIDS

Experiment tells us that a solid substance floats in a liquid denser
than itself, sinks in one less dense, and remains suspended at any
level in one of precisely the same density. If the stone be only
slightly less dense than the liquid, it will rise to the surface; if
it be just as slightly denser, it will as surely sink to the bottom,
a physical fact which has added so much to the difficulty and danger
of submarine manœuvring. If then we can find a liquid denser than the
stone to be tested, and place the latter in it, the stone will float on
the surface. If we take a liquid which is less dense than the stone and
capable of mixing with the heavier liquid, and add it to the latter,
drop by drop, gently stirring so as to assure that the density of
the combination is uniformly the same throughout, a stage is finally
reached when the stone begins to move downwards. It has now very nearly
the density of the liquid, and, if we find by some means this density,
we know simultaneously the specific gravity of the stone.

Various devices and methods are available for ascertaining the density
of liquids—for instance, Westphal’s balance; but, apart from the
inconvenience attending such a determination, the density of all
liquids is somewhat seriously affected by changes in the temperature,
and it is therefore better to make direct comparison with fragments
of substances of known specific gravity, which are termed indicators.
If of two fragments differing slightly in specific gravity one floats
on the surface of a uniform column of liquid and the other lies at
the bottom of the tube containing the liquid, we may be certain that
the density of the liquid is intermediate between the two specific
gravities. Such a precaution is necessary because, if the liquid be a
mixture of two distinct liquids, the density would tend to increase
owing to the greater volatility of the lighter of them, and in any case
the density is affected by change of temperature. The specific gravity
of stones is not much altered by variation in the temperature.

A more convenient variation of this method is to form a diffusion
column, so that the density increases progressively with the depth.
If the stone under test floats at a certain level in such a column
intermediate between two fragments of known specific gravity, its
specific gravity may be found by elementary interpolation. To form a
column of this kind the lighter liquid should be poured on to the top
of the heavier. Natural diffusion gives the most perfect column, but,
being a lengthy process, it may conveniently be quickened by gently
shaking the tube, and the column thus formed gives results sufficiently
accurate for discriminative purposes.

By far the most convenient liquid for ordinary use is methylene
iodide, which has already been recommended for its high refraction.
It has, when pure, a density at ordinary room-temperatures of 3·324,
and it is miscible in all proportions with benzol, whose density is
O·88, or toluol, another hydrocarbon which is somewhat less volatile
than benzol, and whose density is about the same, namely, 0·86. When
fresh, methylene iodide has only a slight tinge of yellow, but it
rapidly darkens on exposure to light owing to the liberation of iodine
which is in a colloidal form and cannot be removed by filtration.
The liquid may, however, be easily cleared by shaking it up with any
substance with which the iodine combines to form an iodide removable
by filtration. Copper filings answer the purpose well, though rather
slow in action; mercury may also be used, but is not very satisfactory,
because a small amount may be dissolved and afterwards be precipitated
on to the stone under test, carrying it down to the bottom of the tube.
Caustic potash (potassium hydroxide) is also recommended; in this case
the operation should preferably be carried out in a special apparatus
which permits the clear liquid to be drawn off underneath, because
water separates out and floats on the surface. In Fig. 32 three cut
stones, a quartz (_a_), a beryl (_b_), and a tourmaline (_c_) are shown
floating in a diffusion column of methylene iodide and benzol. Although
the beryl is only slightly denser than the quartz, it floats at a
perceptibly lower level. These three species are occasionally found as
yellow stones of very similar tint.

[Illustration: FIG. 32.—Stones of different Specific Gravities floating
in a Diffusion Column of heavy Liquid.]

Various other liquids have been used or proposed for the same
purpose, of which two may be mentioned. The first of them is a
saturated solution of potassium iodide and mercuric iodide in water,
which is known after the discoverer as Sonstadt’s solution. It is a
clear mobile liquid with an amber colour, having at 12° C. a density
of 3·085; it may be mixed with water to any extent, and is easily
concentrated by heating; moreover, it is durable and not subject to
alteration of any kind; on the other hand, it is highly poisonous and
cauterizes the skin, not being checked by albumen; it also destroys
brass-ware by amalgamating the metal. The second is Klein’s solution,
a clear yellow liquid which has at 15° C. a density of 3·28. It
consists of the boro-tungstate of cadmium, of which the formula is
9WO_{3}.B_{2}O_{3}.2CdO.2H_{2}O + 16Aq, dissolved in water, with which
it may be diluted. If the salt be heated, it fuses at 75° C. in its
own water of crystallization to a yellow liquid, very mobile, with a
density of 3·55. Klein’s solution is harmless, but it cannot compare
for convenience of manipulation with methylene iodide.

The most convenient procedure is to have at hand three glass tubes,
fitted with stoppers or corks, to contain liquids of different
densities—

(_a_) Methylene iodide reduced to 2·7; using as indicators orthoclase
2·55, quartz 2·66, and beryl 2·74.

(_b_) Methylene iodide reduced to 3·1; indicators, beryl 2·74 and
tourmaline 3·10.

(_c_) Methylene iodide, undiluted, 3·32.

The pure liquid in the last tube should on no account be diluted; but
the density of the other two liquids may be varied slightly, either
by adding benzol in order to lower it, or by allowing benzol, which
has far greater volatility than methylene iodide, to evaporate, or by
adding methylene iodide, in order to increase it. The density of the
liquids may be ascertained approximately from the indicators.

A glance at the table of specific gravities shows that as regards the
gem-stones methylene iodide is restricted in its application, since
it can be used to test only moonstone, quartz, beryl, tourmaline, and
spodumene; opal and turquoise, being amorphous and more or less porous,
should not be immersed in liquids, lest the appearance of the stone be
irretrievably injured. Methylene iodide readily serves to distinguish
the yellow quartz from the true topaz, with which jewellers often
confuse it, the latter stone sinking in the liquid; again aquamarine
floats, but the blue topaz, which is often very similar to it, sinks in
methylene iodide.

By saturating methylene iodide with iodine and iodoform, we have a
liquid (_d_) of density 3·6; a fragment of topaz, 3·55, may be used to
indicate whether the liquid has the requisite density. Unfortunately
this saturated solution is so dark as to be almost opaque, and is,
moreover, very viscous. Its principal use is to distinguish diamond,
3·535, from the brilliant colourless zircon, with which, apart from
a test for hardness, it may easily be confused. It is easy to see
whether the stone floats, as it would do if a diamond. To recover a
stone which has sunk, the only course is to pour off the liquid into
another tube, because it is far too dark for the position of the stone
to be seen.

It is possible to employ a similar method for still denser stones by
having recourse to Retgers’s salt, silver-thallium nitrate. This double
salt is solid at ordinary room-temperatures, but has the remarkable
property of melting at a temperature, 75° C., which is well below the
point of fusion of either of its constituents, to a clear, mobile
yellow liquid, which is miscible in any proportion with water, and
has, when pure, a density of 4·6. The salt may be purchased, or it may
be prepared by mixing 100 grams of thallium nitrate and 64 grams of
silver nitrate, or similar proportions, in a little water, and heating
the whole over a water-bath, keeping it constantly stirred with a
glass rod until it is liquefied. The two salts must be mixed in the
correct proportions, because otherwise the mixture might form other
double salts, which do not melt at so low a temperature. A glance at
the table of specific gravities shows that Retgers’s salt may be used
for all the gem-stones with the single exception of zircon (b). There
are, however, some objections to its use. It is expensive, and, unless
kept constantly melted, it is not immediately available. It darkens on
exposure to strong sunlight like all silver salts, stains the skin a
peculiar shade of purple which is with difficulty removed, and in fact
only by abrasion of the skin, and, like all thallium compounds, is
highly poisonous.

It is convenient to have three tubes, fitted as before with stoppers
or corks, to contain the following liquids, when heated:—

(_e_) Silver-thallium nitrate, reduced to 3·5; using as indicators,
peridot or idocrase 3·40 and topaz 3·53.

(_f_) Silver-thallium nitrate, reduced to 4·0; indicators, topaz 3·53
and sapphire 4·03.

(_g_) Silver-thallium nitrate, undiluted, 4·6.

The tubes must be heated in some form of water-bath; an ordinary glass
beaker serves the purpose satisfactorily. The pure salt should never be
diluted; but the density of the contents of tubes (_e_) and (_f_) may
be varied at will, water being added in order to lower the density, and
concentration by means of evaporation or addition of the nitrate being
employed in order to increase it. To avoid the discoloration of the
skin, rubber finger-stalls may be used, and the stones should not be
handled until after they have been washed in warm water. The staining
may be minimized if the hands be well washed in hot water before being
exposed to sunlight. It is advisable to warm the stone to be tested
in a tube containing water beforehand lest the sudden heating develop
cracks. A piece of platinum, or, failing that, copper wire is of
service for removing stones from the tubes; a glass rod, spoon-shaped
at one end, does equally well. It must be noted that although Retgers’s
salt is absolutely harmless to the ordinary gem-stones—with the
exception of opal and turquoise, which, as has already been stated,
being to some extent porous, should not be immersed in liquids—it
attacks certain substances, for instance, sulphides and cannot be
applied indiscriminately to minerals.

The procedure described above is intended only as a suggestion;
the method may be varied to any extent at will, depending upon the
particular requirements. If such tests are made only occasionally, a
smaller number of tubes may be used. Thus one tube may be substituted
for the two marked _a_ and _b_, the liquid contained in it being
diluted as required, and a series of indicators may be kept apart in
small glass tubes. On the other hand, any one having constantly to test
stones might increase the number of tubes with advantage, and might
find it useful to have at hand fragments of all the principal species
in order to make direct comparison.

(2) DIRECT WEIGHING

The balance which is necessary in both the methods described under this
head should be capable of giving results accurate to milligrams, _i.e._
the thousandth part of a gram, and consistent with that restriction
the beam may be as short as possible so as to give rapid swings and
thus shorten the time taken in the observations. A good assay balance
answers the purpose admirably. Of course, it is never necessary to wait
till the balance has come to rest. The mean of the extreme readings of
the pointer attached to the beam will give the position in which it
would ultimately come to rest. Thus, if the pointer just touches the
eighth division on the right-hand side and the second on the other,
the mean position is the third division on the right-hand side (½(8 −
2) = 3). Instead of the ordinary form of chemical balance, Westphal’s
form or Joly’s spring-balance may be employed. Weighings are made more
quickly, but are not so accurate.

In refined physical work the practice known as double-weighing is
employed to obviate any slight error there may be in the suspension
of the balance. A counterpoise which is heavier than anything to be
weighed is placed in one pan, and weighed. The counterpoise is retained
in its pan throughout the whole course of the weighings. Any substance
whose weight is to be found is placed in the other pan, and weights
added till the balance swings truly again. The difference between
the two sets of weights evidently gives the weight of the substance.
Balances, however, are so accurately constructed that for testing
purposes such refined precautions are not really necessary.

It is immaterial in what notation the weighings are made, so long as
the same is used throughout, but the metric system of weights, which
is in universal use in scientific work, should preferably be employed.
Jewellers, however, use carat weights, and a subdivision to the base
2 instead of decimals, the fractions being ½, ¼, ⅛, 1/16, 1/32, 1/64.
If these weights be employed, it will be necessary to convert these
fractions into decimals, and write ½ = ·500, ¼ ·250, ⅛ = ·125, 1/16 =
·062, 1/32 = ·031, 1/64 = ·016.

(a) _Hydrostatic Weighing_

The principle of this method is very simple. The stone, the specific
gravity of which is required, is first weighed in air and then when
immersed in water. If _W_ and _W´_ be these weights respectively, then
_W_ − _W´_ is evidently the weight of the water displaced by the stone
and having therefore the same volume as it, and the specific gravity is
_W_
therefore equal to ———————————.
_W_ − _W^r_

If the method of double-weighing had been adopted, the formula would
be slightly altered. Thus, suppose that _c_ corresponds to the
counterpoise, _w_ and _w´_ to the stone weighed in air and water
respectively; then we have _W_ = _c_ − _w_ and _W´_ = _c_ − _w´_, and
_c_ − _w_
therefore the specific gravity is equal to ——————————.
_w´_ − _w_

[Illustration: FIG. 33.—Hydrostatic Balance.]

Some precautions are necessary in practice to assure an accurate
result. A balance intended for specific gravity work is provided with
an auxiliary pan (Fig. 33), which hangs high enough up to permit of
the stone being suspended underneath. The weight of anything used for
the suspension must, of course, be determined and subtracted from the
weight found for the stone, both when in air and when in water. A piece
of fine silk is generally used for suspending the stone in water,
but it should be avoided, because the water tends to creep up it and
the error thus introduced affects the first place of decimals in the
case of a one-carat stone, the value being too high. A piece of brass
wire shaped into a cage is much to be preferred. If the same cage be
habitually used, its weight in air and when immersed in water to the
customary extent in such determinations should be found once for all.

Care must also be taken to remove all air-bubbles which cling to the
stone or the cage; their presence would tend to make the value too low.
The surface tension of water which makes it cling to the wire prevents
the balance swinging freely, and renders it difficult to obtain a
weighing correct to a milligram when the wire dips into water. This
difficulty may be overcome by substituting a liquid such as toluol,
which has a much smaller surface tension.

As has been stated above, the density of water at 4° C. is taken as
unity, and it is therefore necessary to multiply the values obtained
by the density of the liquid, whatever it be, at the temperature of
the observation. In Table IX, at the end of the book, are given the
densities of water and toluol at ordinary room-temperatures. It will be
noticed that a correct reading of the temperature is far more important
in the case of toluol.

_Example of a Hydrostatic Determination of Specific Gravity—_

Weight of stone in air = 1·471 gram
Weight of stone in water = 1·067 „
1·471 1·471
Specific gravity = ————————————— = —————
1·471 − 1·067 0·404.

Allowing for the density of water at the temperature of the room, which
was 16° C., the specific gravity is 3·637. Had no such allowance been
made, the result would have been four units too high in the third place
of decimals. For discriminative purposes, however, such refinement is
unnecessary.

(b) _Pycnometer, or Specific Gravity Bottle_

The specific gravity bottle is merely one with a fairly long neck on
which a horizontal mark has been scratched, and which is closed by
a ground glass stopper. The pycnometer is a refined variety of the
specific gravity bottle. It has two openings: the larger is intended
for the insertion of the stone and the water, and is closed by a
stopper through which a thermometer passes, while the other, which is
exceedingly narrow, is closed by a stopper fitting on the outside, and
is graduated to facilitate the determination of the height of the water
in it.

The stone is weighed as in the previous method. The bottle is then
weighed, and filled with water up to the mark and weighed again. The
stone is now introduced into the bottle, and the surplus water removed
with blotting-paper or otherwise until it is at the same level as
before, and the bottle with its contents is weighed. Let _W_ be the
weight of the stone, _w_ the weight of the bottle, _W_´ the weight of
the bottle and the water contained in it, and _W_″ the weight of the
bottle when containing the stone and the water. Then _W_´ − _w_ is the
weight of the water filling the bottle up to the mark, and
_W_″ − _w_ − _W_ is the reduced weight of water after the stone has
been inserted; the difference, _W_ + _W_´ − _W_″, is the weight of the
_W_
water displaced. The specific gravity is therefore ——————————————————.
_W_ + _W_´ − _W_″
As in the previous method, this value must be multiplied by the density
of the liquid at the temperature of the experiment. If the method of
double-weighing be adopted, the formula will be slightly modified.

• • • • •

Of the above methods, that of heavy liquids, as it is usually termed,
is by far the quickest and the most convenient for stones of ordinary
size, the specific gravity of which is less than the density of pure
methylene iodide, namely, 3·324, and by its aid a value may be obtained
which is accurate to the second place of decimals, a result quite
sufficient for a discriminative test. The method is applicable no
matter how small the stone may be, and, indeed, for very small stones
it is the only trustworthy method; for large stones it is inconvenient,
not only because of the large quantity of liquid required, but also
on account of the difficulty in estimating with sufficient certainty
the position of the centre of gravity of the stone. A negative
determination may be of value, especially if attention be paid to the
rate at which the stone falls through the liquid; the denser the stone
the faster it will sink, but the rate depends also upon the shape of
the stone. Retgers’s salt is less convenient because of the delay
involved in warming it and of the almost inevitable staining of the
hands, but its use presents no difficulty whatever.

Hydrostatic weighing is always available, unless the stone be very
small, but the necessary weighings occupy considerable time, and care
must be taken that no error creeps into the computation, simple though
it be. Even if everything is at hand, a determination is scarcely
possible under a quarter of an hour.

The third method, which takes even longer, is intended primarily for
powdered substances, and is not recommended for cut stones, unless
there happen to be a number of tiny ones which are known to be exactly
of the same kind.

The specific gravities of the gem-stones are given in Table VII at the
end of the book.

CHAPTER IX

HARDNESS AND CLEAVABILITY

Every possessor of a diamond ring is aware that diamond easily
scratches window-glass. If other stones were tried, it would be
found that they also scratched glass, but not so readily, and, if
the experiment were extended, it would be found that topaz scratches
quartz, but is scratched by corundum, which in its turn yields to
the all-powerful diamond. There is therefore considerable variation
in the capacity of precious stones to resist abrasion, or, as it is
usually termed, in their hardness. To simplify the mode of expressing
this character the mineralogist Mohs about a century ago devised the
following arbitrary scale, which is still in general use.

MOHS’S SCALE OF HARDNESS

1. Talc
2. Gypsum
3. Calcite
4. Fluor
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond

A finger-nail scratches gypsum and softer substances. Ordinary
window-glass is slightly softer than orthoclase, and a steel knife is
slightly harder; a hardened file approaches quartz in hardness, and
easily scratches glass.

By saying that a stone has hardness 7 we merely mean that it will not
scratch quartz, and quartz will not scratch it. The numbers indicate
an order, and have no quantitative significance whatever. This is an
important point about which mistakes are often made. We must not, for
instance, suppose that diamond has twice the hardness of apatite.
As a matter of fact, the interval between diamond and corundum is
immensely greater than that between the latter and talc, the softest
of mineral substances. Intermediate degrees of hardness are expressed
by fractions. The number 8½ for chrysoberyl means that it scratches
topaz as easily as it itself is scratched by corundum. Pyrope garnet is
slightly harder than quartz, and its hardness is said therefore to be
7¼.

Delicate tests show that the structure of all crystallized substances
is more or less grained, like that of wood, and the hardness for the
same stone varies in different directions. Kyanite is unique in this
respect, since its hardness ranges from 5 to 7; it can therefore
be scratched by a knife in some directions, but not in others. In
most substances, however, the range is so small as to be quite
imperceptible. Slight variation is also apparent in the hardness of
different specimens of the same species. The diamonds from Borneo and
New South Wales are so distinctly harder than those from South Africa
and other localities that, when first discovered, some difficulty was
experienced in cutting them. Again, lapidaries find that while Ceylon
sapphires are harder than rubies, Kashmir sapphires are softer.

Hardness is a character of fundamental importance in a stone intended
for ornamental wear, since upon it depends the durability of the polish
and brilliancy. Ordinary dust is largely composed of grains of sand,
which is quartz in a minute form, and a gem-stone should therefore
be at least as hard as that. Paste imitations are little harder than
5, and consequently, as experience shows, their polish does not
survive a few weeks’ wear. Hardness is, however, of little use as a
discriminative test except for distinguishing between topaz or harder
stone and paste. Diamond is so much harder than other stones that it
will leave a cut in glass quite different from the scratch of even
corundum. Paste, being so soft, readily yields to the file, and is thus
easily distinguished from genuine stones. In applying the test to a
cut stone, it is best to remove it from its mount and try the effect
on the girdle, because any scratch would be concealed afterwards by
the setting. Any mark should be rubbed with the finger to assure that
it is not due to powder from the scratching agent; confusion may often
be caused in this way when the two substances are of nearly the same
hardness.

The degrees of hardness of the gem-stones are given in Table VIII at
the end of the book.

• • • • •

It must not be overlooked that extreme hardness is compatible with
cleavability in certain directions intimately connected with the
crystalline structure; the property, in fact, characterizes many
mineral species of different degrees of hardness. Diamond can be split
in four directions parallel to the faces of the regular octahedron,
a property utilized by the lapidary for shaping a stone previous to
cutting it. Topaz cleaves with considerable ease at right angles to the
principal crystallographic axis. Felspar has two directions of cleavage
nearly at right angles to one another. The new gem-stone, kunzite,
needs cautious handling owing to the facility with which it splits in
two directions mutually inclined at about 70°.

All stones are more or less brittle, and will be fractured by a
sufficiently violent blow, but the irregular surface of a fracture
cannot be mistaken for the brilliant flat surface given by a cleavage.
The cleavage is by no means induced with equal facility in the species
mentioned above. A considerable effort is required to split diamond,
but in the case of topaz or kunzite incipient cleavage in the shape of
flaws may be started if the stone be merely dropped on to a hard floor.

CHAPTER X

ELECTRICAL CHARACTERS

The definite orientation of the molecular arrangement of crystallized
substances leads in many cases to attributes which vary with the
direction and are revealed by the electrical properties. If a
tourmaline crystal be heated in a gas or alcohol flame it becomes
charged with electricity, and, since it is at the same time a bad
conductor, static charges of opposite sign appear at the two ends.
Topaz shows similar characters, but in a lesser degree. Quartz, if
treated in the same way, shows charges of opposite sign on different
sides, but the phenomenon may be masked by intimate twinning and
consequent overlapping of the contrary areas. The phenomenon may
also be seen when the stones are cut. The most convenient method for
detecting the existence of the electrical charges is that devised
by Kundt. A powder consisting of a mixture of red lead and sulphur
is placed in a bellows arrangement and blown through a sieve at one
end on to the stone. Owing to the friction the particles become
electrified—red lead positively and sulphur negatively—and are
attracted by the charges of opposing sign, which will therefore be
betrayed by the colour of the dust at the corresponding spot. The
powder must be kept dry; otherwise a chemical reaction may occur
leading to the formation of lead sulphide, recognizable by its black
colour. Bücker has suggested as an alternative the use of sulphur,
coloured red with carmine, the negative element, and yellow lycopodium,
the positive element.

Diamond, topaz, and tourmaline are powerful enough, when electrified
by friction with a cloth, to attract fragments of paper, the
electrification being positive. Amber develops considerable negative
electricity when treated in a similar manner.

Diamond is translucent to the Röntgen (X) rays; glass, on the other
hand, is opaque to them, and this test distinguishes brilliants from
paste imitations. Diamond also, unlike glass, phosphoresces under the
influence of radium, a property characterizing also kunzite.

It will be seen that the electrical characters, although of
considerable interest to the student, are, on account of their
limited application and difficulty of test, of little service for the
discrimination of gem-stones.

PART I—SECTION B

THE TECHNOLOGY OF GEM-STONES

CHAPTER XI

UNIT OF WEIGHT

The system in use for recording the weights of precious stones is
peculiar to jewellery. The unit, which is known as the carat, bears no
simple relation to any unit that has existed among European nations,
and indubitably has been introduced from the East. When man in early
days sought to record the weights of small objects, he made use of the
most convenient seeds or grains which were easily obtainable and were
at the same time nearly uniform in size. In Europe the smallest unit of
weight was the barley grain. Similarly in the East the seeds of some
leguminous tree were selected. Those of the locust-tree, _Ceratonia
siliqua_, which is common in the countries bordering the Mediterranean,
on the average weigh so nearly a carat that they almost certainly
formed the original unit. It is, indeed, from the Greek κεράτιον,
little horn, which refers to the shape of the pods, that the word carat
is derived.

It is one of the eccentricities of the jewellery trade that precision
should not have been given to the unit of weight. Not only does it
vary at most of the trade centres in the world, but it is not even
always constant at each centre. The difference is negligible in the
case of single stones of ordinary size, but becomes a matter of serious
importance when large stones, or parcels of small stones, are bought
and sold, particularly when the stones are very costly. Attempts have
been made at various times to secure a uniform standard, but as yet
with only partial success. In 1871 the carat defined as the equivalent
of 0·20500 gram was suggested at a meeting of the principal jewellers
of Paris and London, and was eventually accepted in Paris, New York,
Leipzig, and Borneo. It has, however, recently been recognized that
in view of the gradual spread of the metric system of weights and
measures the most satisfactory unit is the metric carat of one-fifth
(0·2) gram. This has now been constituted the legal carat of France
and Belgium, and no doubt other countries will follow their example.
The carat weight obtaining in London weighs about 0·20530 gram, and
the approximate equivalents in the gram at other centres are as
follows:—Florence 0·19720, Madrid 0·20539, Berlin 0·20544, Amsterdam
0·20570, Lisbon 0·20575, Frankfort-on-Main 0·20577, Vienna 0·20613,
Venice 0·20700, and Madras 0·20735. The gram itself is inconveniently
large to serve as a unit for the generality of stones met with in
ordinary jewellery.

The notation for expressing the sub-multiples of the carat forms
another curious eccentricity. Fractions are used which are powers of
the half: thus the half, the half of that, _i.e._ the quarter, and so
on down to the sixty-fourth, and the weight of a stone is expressed
by a series of fractions, _e.g._ 3½ ⅛ 1/64 carats. In the case of
diamond a single unreduced fraction to the base 64 is substituted in
place of the series of single fractions, and the weight of a stone is
stated thus, 4-40/64 carats. With the introduction of the metric
carat the more convenient and rational decimal notation would, of
course, be simultaneously adopted.

[Illustration: Figs. 34-39.—Exact Sizes of Brilliants of various
Weights.]

Figs. 34-39 illustrate the exact sizes of diamonds of certain weights,
when cut as brilliants. The sizes of other stones depends upon their
specific gravity, the weight varying as the volume multiplied by the
specific gravity. Quartz, for instance, has a low specific gravity and
would be perceptibly larger, weight for weight; zircon, on the other
hand, would be smaller.

It has been found more convenient to select a smaller unit in the case
of pearls, namely, the pearl-grain, four of which go to the carat.

Stencil gauges are in use for measuring approximately the weight in
carats of diamond brilliants and of pearls, which in both instances
must be unmounted. A more accurate method for determining the weight
of diamonds has been devised by Charles Moe, which is applicable to
either unmounted or mounted stones. By means of callipers, which read
to three-tenths of a millimetre, the diameter and the depth of the
stone are measured, and by reference to a table the corresponding
weight is found; allowance is made for the varying fineness of the
girdle, and, in the case of large stones, for the variation from a
strictly circular section.

• • • • •

Since this chapter was written the movement in favour of the metric
carat has made rapid progress, and this unit will soon have been
adopted as the legal standard all over the world, even in countries,
such as the British Isles and the United States, where the metric
system is not in use. The advantage of an international unit is too
obvious to need arguing.

CHAPTER XII

FASHIONING OF GEM-STONES

Although many of the gem-stones have been endowed by nature with
brilliant lustrous faces and display scintillating reflections from
their surfaces, yet their form is never such as to reveal to full
perfection the optical qualities upon which their charm depends.
Moreover, the natural faces are seldom perfect; as a rule the stones
are broken either through some convulsion of the earth’s crust or in
course of extraction from the matrix in which they have lain, or they
are roughened by attrition against matter of greater hardness, or worn
by the prolonged action of water, or etched by solvents. Beautiful
octahedra of diamond or spinel have been mounted without further
embellishment, but even their appearance might have been much improved
at the lapidary’s hands.

By far the oldest of the existing styles of cutting is the rounded
shape known as cabochon, a French word derived from the Latin _cabo_,
a head. In the days of the Roman Empire the softer stones were often
treated in this manner; such stones were supposed to be beneficial
to those suffering from short-sightedness, the reason no doubt being
that transparent stones when cut as a double cabochon formed a convex
lens. According to Pliny, Nero had an emerald thus cut, through
which he was accustomed to view the gladiatorial shows. This style of
cutting was long a favourite for coloured stones, such as emerald,
ruby, sapphire, and garnet, but has been abandoned in modern practice
except for opaque, semi-opaque, and imperfect stones. The crimson
garnet, which was at one time known by the name carbuncle, was so
systematically thus cut that the word has come to signify a red garnet
of this form. It was a popular brooch-stone with our grandmothers, but
is no longer in vogue. The East still retains a taste for stones cut
in the form of beads and drilled through the centre; the beads are
threaded together, and worn as necklaces. The native lapidaries often
improve the colour of pale emeralds by lining the hole with green paint.

[Illustration: _PLATE V_

JEWELLERY DESIGNS]

[Illustration: FIG. 40.—Double (Convex) Cabochon.]

[Illustration: FIG. 41.—Simple Cabochon.]

The cabochon form may be of three different kinds. In the first, the
double cabochon (Fig. 40), both the upper and the under sides of the
stones are curved. The curvature, however, need not be the same in
each case; indeed, it is usually markedly different. Moonstones and
starstones are generally cut very steep above and shallow underneath.
Occasionally a ruby or a sapphire is, when cut in this way, set with
the shallow side above, because the light that has penetrated into the
stone from above is more wholly reflected from a steep surface with
consequent increase in the glow of colour from the stone. Opals are
always cut higher on the exposed side, but the slope of the surface
varies considerably; they are generally cut steeply when required for
mounting in rings. Chrysoberyl cat’s-eyes are invariably cut with
curved bases in order to preserve the weight as great as possible.
The double cabochon form with a shallow surface underneath merges
into the second kind (Fig. 41) in which the under side is plane, the
form commonly employed for quartz cat’s-eyes, and occasionally also
for carbuncles. In this type the plane side is invariably mounted
downwards. In the third form (Fig. 42) the curvature of the under
surface is reversed, and the stone is hollowed out into a concave
shape. This style is reserved for dark stones, such as carbuncles,
which, if cut at all thick, would show very little colour. A piece of
foil is often placed in the hollow in order to increase the reflection
of light, and thus to heighten the colour effect.

[Illustration: FIG. 42.—Double (Concavo-convex) Cabochon.]

In early days it was supposed that the extreme hardness of diamond
precluded the possibility of fashioning it, and up to the fifteenth
century all that was done was to remove the gum-like skin which
disfigured the Indian stones and to polish the natural facets.
The first notable advance was made in 1475, when Louis de Berquem
discovered, as it is said quite by accident, that two diamonds if
rubbed together ground each other. With confident courage he essayed
the new art upon three large stones entrusted to him by Charles the
Bold, to the entire satisfaction of his patron. The use of wheels
or discs charged with diamond dust soon followed, but at first the
lapidaries evinced their victory over such stubborn material by
grinding diamond into divers fantastic shapes, and failed to realize
how much might be done to enhance the intrinsic beauty of the stones by
the means now at their disposal. The Indian lapidaries arrived at the
same discovery independently, and Tavernier found, when visiting the
country in 1665, a large number of diamond cutters actively employed.
If the stone were perfectly clear, they contented themselves with
polishing the natural facets; but if it contained flaws or specks, they
covered it with numerous small facets haphazardly placed. The stone was
invariably left in almost its original shape, and no effort was made to
improve the symmetry.

[Illustration: FIG. 43.—Table Cut (top view).]

[Illustration: FIG. 44.—Table Cut (side view).]

For a long time little further progress was made, and even nearly a
century after Berquem the only regular patterns known to Kentmann, who
wrote in 1562, were the diamond-point and the diamond-table (Figs.
43-44). The former consisted of the natural octahedron facets ground
to regular shape, and was long employed for the minute stones which
were set in conjunction with large coloured stones in rings. The table
represented considerably greater labour. One corner of the regular
octahedron was ground down until the artificial facet thus produced was
half the width of the stone, while the opposite corner was slightly
ground.

Still another century elapsed before the introduction of the rose
pattern, which comprised twenty-four triangular facets and a flat base
(Figs. 45-46), the stone being nearly hemispherical in shape. This
style is said to have been the invention of Cardinal Mazarin, but
probably he was the first to have diamonds of any considerable size cut
in this form. At the present day only tiny stones are cut as roses.

[Illustration: FIG. 45.—Rose Cut (top view).]

A few more years passed away, and at length at the close of the
seventeenth century diamond came by its own when Vincenzio Peruzzi, a
Venetian, introduced the brilliant form of cutting, and revealed for
the first time its amazing ‘fire.’ Except for minor changes this form
remains to this day the standard style for the shape of diamond, and
the word brilliant is commonly employed to denote diamond cut in this
way. So obviously and markedly superior is the style to all others
that upon its discovery the owners of large roses had them re-cut as
brilliants despite the loss in weight necessitated by the change.

[Illustration: FIG. 46.—Rose Cut (side view).]

The brilliant form is derived from the old table by increasing the
number of facets and slightly altering the angles pertaining to the
natural octahedron. In a perfect brilliant (Figs. 47-49) there are
altogether 58 facets, 33 above and 25 below the girdle, as the edge
separating the upper and lower portions of the stone is termed, which
are arranged in the following manner. Eight star-facets, triangular
in shape, immediately surround the large table-facet. Next come four
large templets or bezels, quadrilateral in form, arranged in pairs
on opposite sides of the table-facet, the four quoins or lozenges,
similar in shape, coming intermediately between them; in modern
practice, however, these two sets are identical in shape and size,
and there are consequently eight facets of the same kind instead of
two sets of four. The eight cross or skew facets and the eight skill
facets, in both sets the shape being triangular, form the boundary
of the girdle; modern brilliants usually have instead sixteen facets
of the same shape and size. The above 33 facets lie above the girdle
and form the crown of the stone. Immediately opposite and parallel to
the table is the tiny culet. Next to the latter come the four large
pavilion facets with the four quoins intermediately between them,
both sets being five-sided but nearly quadrilateral in shape; these
again are usually combined into eight facets of the same size. Eight
cross facets and eight skill facets, both sets, like those in the
crown, being triangular in shape, form the lower side of the girdle;
these also are generally united into a set of sixteen similar facets.
These 25 facets which lie below the girdle comprise the ‘pavilion,’
or base of the stone. In a regular stone properly cut a templet is
nearly parallel to a pavilion, and an upper to a lower cross facet. The
contour of the girdle is usually circular, but occasionally assumes
less symmetrical shapes, as for instance in drop-stones or pendeloques,
and the facets are at the same time distorted. The number of facets may
with advantage be increased in the case of large stones. An additional
set of eight star facets is often placed round the culet, the total
number then being 66. It may be mentioned that the largest stone cut
from the Cullinan has the exceptional number of 74 facets.

[Illustration: FIG. 47.—Brilliant Cut (top view).]

[Illustration: FIG. 48.—Brilliant Cut (base view).]

[Illustration: FIG. 49.—Brilliant Cut (side view).]

In order to secure the finest optical effect certain proportions have
been found necessary. The depth of the crown must be one-half that of
the base, and therefore one-third the total depth of the stone, and the
width of the table must be slightly less than half that of the stone.
The culet should be quite small, not more in width than one-sixth of
the table; it is, in fact, not required at all except to avoid the
danger of the point splintering. The girdle should be as thin as is
compatible with strength sufficient to prevent chipping in the process
of mounting the stone; if it were left thick, the rough edge would be
visible by reflection at the lower facets, and would, especially if at
all dirty, seriously affect the quality of the stone. The shape of the
stone is largely determined by the sizes of the templets in the crown
and the pavilions in the base as compared with that of the table, or,
what comes to the same thing, by the inclinations at which they are cut
to that facet. If the table had actually half the width of the stone,
the angle[5] between it and a templet would be exactly half a right
angle or 45°; it is, however, made somewhat smaller, namely, about 40°.
A pavilion, being parallel to a templet, makes a similar angle with the
culet. The cross facets are more steeply inclined, and make an angle
of about 45° with the table or the culet, as the case may be. The star
facets, on the other hand, slant perceptibly less, and make an angle of
only about 26° with the table. A latitude of some 4° or 5° is possible
without seriously affecting the ‘fire’ of the stone.

The object of the disposition of the facets on a brilliant is to assure
that all the light that enters the stone, principally by way of the
table, is wholly reflected from the base and emerges through the crown,
preferably by way of the inclined facets. A brilliant-cut diamond, if
viewed with the table between the observer and the light, appears quite
dark except for the small amount of light escaping through the culet.
Light should therefore fall on the lower facets at angles greater
than the critical angle of total-reflection, which for diamond is 24°
26´. The pavilions should be inclined properly at double this angle,
or 48° 52´, to the culet; but a ray that emerges at a pavilion in the
actual arrangement entered the table at nearly grazing incidence, and
the amount of light entering this facet at such acute perspective is
negligible. On the other hand, after reflection at the base light must,
in order to emerge, fall on the crown at less than the critical angle
of total-reflection. In Fig. 50 are shown diagrammatically the paths
of rays that entered the table in divers ways. The ray emerging again
at the table suffers little or no dispersion and is almost white,
but those coming out through the inclined facets are split up into
the rainbow effect, known as ‘fire,’ for which diamond is so famous.
It is in order that so much of the light entering by the table may
emerge through the inclined facets of the crown that the pavilions are
inclined at not much more than 40° to the culet. It might be suggested
that instead of being faceted the stone should be conically shaped,
truncated above and nearly complete below. The result would no doubt
be steadier, but, on the other hand, far less pleasing. It is the
ever-changing nuance that chiefly attracts the eye; now a brilliant
flash of purest white, anon a gleam of cerulean blue, waxing to
richest orange and dying in a crimson glow, all intermingled with the
manifold glitter from the surface of the stone. Absolute cleanliness is
essential if the full beauty of any stone is to be realized, but this
is particularly true of diamond. If the back of the stone be clogged
with grease and dirt, as so often happens in claw-set rings, light is
no longer wholly reflected from the base; much of it escapes, and the
amount of ‘fire’ is seriously diminished.

[Illustration: FIG. 50.—Course of the Rays of Light passing through a
Brilliant.]

Needless to state, lapidaries make no careful angular measurements
when cutting stones, but judge of the position of the facets entirely
by eye. It sometimes therefore happens that the permissible limits
are overstepped, in which event the stone is dead and may resist all
efforts to vivify it short of the heroic course of re-cutting it, too
expensive a treatment in the case of small stones.

The factors that govern the properties of a brilliant-cut stone are
large colour-dispersion, high refraction, and freedom from any trace of
intrinsic colour. The only gem-stone that can vie with diamond in these
respects is zircon. Although it is rare to find a zircon naturally
without colour, yet many kinds are easily deprived of their tint by
the application of heat. A brilliant-cut zircon is, indeed, far from
readily distinguished by eye from diamond, and has probably often
passed as one, but it may easily be identified by its large double
refraction (cf. p. 41) and inferior hardness. The remaining colourless
stones, such as white sapphire, topaz, and quartz (rock-crystal), have
insufficient refractivity to give total-reflection at the base, and,
moreover, they are comparatively deficient in ‘fire.’

[Illustration: FIG. 51.—Step- or Trap-Cut (top view).]

[Illustration: FIG. 52.—Step- or Trap-Cut (side view).]

A popular style of cutting which is much in vogue for coloured stones
is the step- or trap-cut, consisting of a table and a series of
facets with parallel horizontal edges (Figs. 51-52) above and below
the girdle; in recent jewellery, however, the top of the stone is
often brilliant-cut. The contour may be oblong, square, lozenge, or
heart-shaped, or have less regular forms. The table is sometimes
slightly rounded. Since the object of this style is primarily to
display the intrinsic colour of the stone and not so much a brilliant
play of light from the interior, no attempt is made to secure
total-reflection at the lower facets. The stone therefore varies in
depth according to its tint; if dark, it is cut shallow, lest light
be wholly absorbed within, and the stone appear practically opaque,
but if light, it is cut deep, in order to secure fullness of tint.
Much precision in shape and disposition of the facets is not demanded,
and the stones are usually cut in such a way that, provided the
desired effect is obtained, the weight is kept as great as possible;
we may recall that stones are sold by weight. In considering what
will be the optical effect of any particular shape, regard must be
had to the effective colour of the transmitted light. For instance,
although sapphire and ruby belong to the same species and have the
same refractive indices, yet, since the former transmits mainly blue
and the latter red light, they have for practical purposes appreciably
different indices, and lapidaries find it therefore possible to cut
the base of ruby thicker than that of sapphire, and thus keep the
weight greater. It is instructive too what can be done with the most
unpromising material by the exercise of a little ingenuity. Thus Ceylon
sapphires are often so irregularly coloured that considerable skill
is called for in cutting them. A stone may, for instance, be almost
colourless except for a single spot of blue; yet, if the stone be
cut steeply and the spot be brought to the base, the effect will be
precisely the same as if the stone were uniformly coloured, because all
the light emerging from the stone has passed through the spot at the
base and therefore been tinted blue.

The mechanism employed in the fashioning of gem-stones is simple in
character, and comprises merely metal plates or wheels for slitting,
and discs or laps for grinding and polishing the stones, the former
being set vertically and rotated about horizontal spindles, and the
latter set horizontally and rotated about vertical spindles. Mechanical
power is occasionally used for driving both kinds of apparatus, but
generally, especially in slitting and in delicate work, hand-power is
preferred. In the East native lapidaries make use of vertical wheels
(Plate XIII) also for grinding and polishing stones, which explains why
native-cut stones never have truly plane facets; it will be noticed
from the picture that a long bow is used to drive the spindle.

Owing to the unique hardness of diamond it can be fashioned only by
the aid of its own powder. The process differs therefore materially
from the cutting of the remaining gem-stones, and will be described
separately. Indeed, so different are the two classes of work that firms
seldom habitually undertake both.

The discovery of the excellent cleavage of diamond enormously reduced
the labour of cutting large stones. A stone containing a bad flaw may
be split to convenient shape in as many minutes as the days or even
weeks required to grind it down. The improvement in the appliances and
the provision of ample mechanical power has further accelerated the
process and reduced the cost. Two years were occupied in cutting the
diamond known as the Pitt or Regent, whereas in only six months the
colossal Cullinan was shaped into two large and over a hundred smaller
stones with far less loss of material.

Although the brilliant form was derived from the regular octahedron, it
by no means follows that, because diamond can be cleaved to the latter
form, such is the initial step in fashioning the rough mass. The aim of
the lapidary is to cut the largest possible stone from the given piece
of rough, and the finished brilliant usually bears no relation whatever
to the natural octahedron. The cleavage is utilized only to free the
rough of an awkward and useless excrescence, or of flaws. Although the
octahedron is one of the common forms in which diamond is found, it is
rarely regular, and oftener than not one of the larger faces is made
the table.

The old method, which is still in use, for roughly fashioning diamonds
is that known as bruting, from the French word, _brutage_, for the
process, or as shaping. Two stones of about the same size are selected,
and are firmly attached by means of a hard cement to the ends of
two holders, which are held one in each hand, and rubbed hard, one
against the other, until surfaces of the requisite size are developed
on each stone. During the process the stones are held over a small
box, which catches the precious powder. A fine sieve at the bottom
of the box allows the powder to fall through into a tray underneath,
but holds back anything larger. By means of two vertical pins placed
one on each side of the box the holders are retained more easily in
the desired position, and the work is thrown mainly on the thumbs.
This work continued day after day has a very disfiguring effect upon
the hands despite the thick gloves that are worn to protect them; the
skin of the thumbs grows hard and horny, and the first and second
fingers become swollen and distorted. When the surfaces have thus been
formed, the stone is handed to the polisher, who works them into the
correct shape and afterwards polishes them, the stone passing backwards
and forwards several times between the cutter and the polisher. The
table, four templets, culet and four pavilions are first formed and
polished, so that the table has a square shape. Next the quoins are
developed and polished, and finally the small facets are polished on,
not being shaped first. In modern practice the process of bruting has
been modified in some cases by the introduction of machinery, and the
facets are ground on, with considerable improvement in the regularity
of their size and disposition, and reduction in the amount of polishing
required. Moreover, to obviate the loss of material resulting from
continued grinding, large stones are first sliced by means of
rapidly-revolving copper wheels charged with diamond powder.

The laps used for polishing diamonds are made of a particular kind
of soft iron, which is found to surpass any other metal in retaining
the diamond powder. They are rotated at a high rate of speed, which
is about 2000 to 2500 revolutions a minute, and the heat developed by
the friction at this speed is too great for a cement to be used; a
solder or fusible alloy, composed of one part tin to three parts lead,
therefore takes its place. The solder is held in a hollow cup of brass
which is from its shape called a ‘dop,’ an old Dutch word meaning
shell. Its external diameter is ordinarily about 1½ in. (4 cm.), but
larger dops are, of course, used for large stones. A stout copper stalk
is attached to the bottom of the dop; it is visible in the view of the
dop shown at _e_ on Plate VI, and two slabs of solder are seen lying in
front of the dop. The dop containing the solder is placed in the midst
of a non-luminous flame and heated until the solder softens, when it is
removed by means of the small tongs, _c_, and placed upright on a stand
such as that shown at _a_. The long tongs, _d_, are used for shaping
the solder into a cone at the apex of which the diamond is placed. The
solder is worked well over the stone so that only the part to undergo
polishing is exposed. A diamond in position is shown at _f_. The top of
the stand is saucer-shaped to catch the stone should it accidentally
fall off the dop, and to prevent pieces of solder falling on the hand.
While still hot, the dop with the diamond in position on the solder is
plunged into cold water in order to cool it. The fact that the stone
withstands this drastic treatment is eloquent testimony to its good
thermal conductivity; other gem-stones would promptly split into
fragments. It may be remarked that so high is the temperature at which
diamond burns that it may be placed in the gas flame without any fear
of untoward results. The dop is now ready for attachment to an arm
such as that shown at _b_; the stalk of the dop is placed in a groove
running across the split end of the arm, and is gripped tight by means
of a screw worked by the nut which is visible in the picture.

[Illustration: _PLATE VI_

APPLIANCES USED FOR POLISHING DIAMONDS.]

[Illustration: _PLATE VII_

POLISHING DIAMONDS]

Four such arms, each with a dop, are used with the polishing lap (Plate
VII), and each stands on two square legs on the bench. Pins, _p_, in
pairs are fixed to the bench to prevent the arms being carried round by
the friction; one near the lap holds the arm not far from the dop, and
the other engages in a strong metal tongue, which is best seen at the
end of the arm _b_ on Plate VI. Though the arm, which is made of iron,
is heavy, yet for polishing purposes it is insufficient, and additional
lead weights are laid on the top of it, as in the case of the arm
at the back on Plate VII. The copper stalk is strong, yet flexible,
and can be bent to suit the position of the facet to be polished; on
Plate VII the dops _a_ and _b_ are upright, but the other two are
inclined. In addition to the powder resulting from bruting, boart,
_i.e._ diamonds useless for cutting, are crushed up to supply polishing
material, and a little olive oil is used as a lubricant. Owing to the
friction so much heat is developed that even the solder would soften
after a time, and therefore, as a precaution, the dop is from time
to time cooled by immersion in water. The stone has constantly to be
re-set, about six being the maximum even of the tiny facets near the
girdle that can be dealt with by varying the inclination of the dop.
As the work approaches completion the stone is frequently inspected,
lest the polishing be carried too far for the development of the proper
amount of ‘fire.’ When finished, the stones are boiled in sulphuric
acid to remove all traces of oil and dirt.

The whole operation is evidently rough and ready in the extreme; but
such amazing skill do the lapidaries acquire, that even the most
careful inspection by eye alone would scarce detect any want of proper
symmetry in a well-cut stone.

The fashioning of coloured stones, as all the gem-stones apart from
diamond are termed in the jewellery trade, is on account of their
inferior hardness a far less tedious operation. They are easily
slit, for which purpose a vertical wheel (Plate VIII) made of soft
iron is used; it is charged with diamond dust and lubricated with
oil, generally paraffin. When slit to the desired size, the stone is
attached to a conveniently shaped holder by means of a cement, the
consistency of which varies with the hardness of the stone. It is
set in the cement in such a way that the plane desired for the table
facet is at right angles to the length of the holder, and the whole of
the upper part or crown is finished before the stone is removed from
the cement. The lower half or base is treated in a similar manner.
Thus in the process of grinding and polishing the stone is only once
re-set; as was stated above, diamond demands very different treatment.
Again, all coloured stones are ground down without any intermediate
operation corresponding to bruting. The holder is merely held in the
hand, but to maintain its position more exactly its other end,
which is pointed, is inserted in one of the holes that are pierced at
intervals in a vertical spindle placed at a convenient distance from
the lap (Plate VIII), which one depending upon the inclination of the
facet to be formed. For hard stones, such as ruby and sapphire, diamond
powder is generally used as the abrasive agent, while for the softer
stones emery, the impure corundum, is selected; in recent years the
artificially prepared carborundum, silicide of carbon corresponding to
the formula CSi, which is harder than corundum, has come into vogue
for grinding purposes, but it is unfortunately useless for slitting,
because it refuses to cling to the wheel. To efface the scratches left
by the abrasive agent and to impart a brilliant polish to the facets,
material of less hardness, such as putty-powder, pumice, or rouge, is
employed; in all cases the lubricant is water. The grinding laps are
made of copper, gun-metal, or lead; and pewter or wooden laps, the
latter sometimes faced with cloth or leather, are used for polishing.
As a general rule, the harder the stone the greater the speed of the
lap.

[Illustration: _PLATE VIII_

SLITTING COLOURED STONES

POLISHING COLOURED STONES]

[Illustration: _PLATE IX_

FACETING MACHINE]

As in the case of diamond, the lapidary judges of the position of the
facet entirely by eye and touch, but a skilled workman can develop
a facet very close to the theoretical position. During recent years
various devices have been invented to enable him to do his work with
greater facility. A machine of this kind is illustrated on Plate IX.
The stone is attached by means of cement to the blunt end, _d_, of
the holder, _b_, which is of the customary kind, while the other end
is inserted in a hole in a wooden piece, _a_, which is adjustable in
height by means of the screw above it. The azimuthal positions of the
facets are arranged by means of the octagonal collar, _c_, the sides
of which are held successively in turn against the guide, _e_. The
stand itself is clamped to the bench. The machine is, however, little
used except for cheap stones, because it is too accurate and leads to
waste of material. Stones are sold by weight, and so long as the eye is
satisfied, no attempt is made to attain to absolute symmetry of shape.

The pictures on Plates X-XIII illustrate lapidaries’ workshops in
various parts of the world. The first two show an office and a workshop
situated in Hatton Garden, London; in the former certain of the staff
are selecting from the parcels stones suitable for cutting. The third
depicts a more primitive establishment at Ekaterinburg in the Urals.
The fourth shows a typical French family—_père_, _mère_, _et fils_—in
the Jura district, all busily engaged; on the table will be noticed a
faceting machine of the kind described above. In the fifth picture a
native lapidary in Calcutta is seen at work with the driving bow in his
right, and the stone in his left, hand.

A curious difference exists in the systems of charging for cutting
diamonds and coloured stones. The cost of cutting the latter is
reckoned by the weight of the finished stone, the rate varying from
1s. to 8s. a carat according to the character of the stone and the
difficulty of the work; while in the case of diamonds, on the other
hand, the weight of the rough material determines the cost, the rate
being about 10s. to 40s. a carat according to the size, which on the
average is equivalent to about 30s. to 120s. a carat calculated on
the weight of the finished stone. The reason of the distinction is
obviously because the proper proportions in a brilliant-cut diamond
must be maintained, whatever be the loss in weight involved; in
coloured stones the shape is not of such primary importance.

[Illustration: _PLATE X_

LAPIDARY’S WORKSHOP AND OFFICE IN ENGLAND]

[Illustration: _PLATE XI_

LAPIDARY’S WORKSHOP IN RUSSIA]

When finished, the stone finds its way with others akin to it to the
manufacturing jeweller’s establishment, where it is handed to the
setter, who mounts it in a ring, necklace, brooch, or whatever article
of jewellery it is intended for. The metal used in the groundwork of
the setting is generally gold, but platinum is also employed where an
unobtrusive and untarnishable metal is demanded, and silver finds a
place in cheaper jewellery, although it is seriously handicapped by
its susceptibility to the blackening influence of the sulphurous fumes
present in the smoke-laden atmosphere of towns. The stone may be either
embedded in the metal or held by claws. The former is by far the safer,
but the latter the more elegant, and it has the advantage of exposing
the stone _à jour_, to use the French jewellers’ expression, so that
its genuineness is more evidently testified. It is very important
that the claw setting be periodically examined, lest the owner one
day experience the mortification of finding that a valuable stone has
dropped out; gold, owing to its softness, wears away in course of time.

Up to quite recent years modern jewellery was justly open to the
criticism that it was lacking in variety, that little attempt was made
to secure harmonious association in either the colour or the lustre of
the gem-stones, and that the glitter of the gold mount was frequently
far too obtrusive. Gold consorts admirably with the rich glow of
ruby, but is quite unsuited to the gleaming fire of a brilliant. Where
the metal is present merely for the mechanical purpose of holding the
stones in position, it should be made as little noticeable as possible.
The artistic treatment of jewellery is, however, receiving now adequate
attention in the best Paris and London houses. Some recent designs are
illustrated on Plates IV and V.

[Illustration: _PLATE XII_

FRENCH FAMILY CUTTING STONES]

[Illustration: _PLATE XIII_

INDIAN LAPIDARY]

CHAPTER XIII

NOMENCLATURE OF PRECIOUS STONES

The names in popular use for the principal gem-stones may be traced
back to very early times, and, since they were applied long before
the determinative study of minerals had become a science, their
significance has varied at different dates, and is even now far from
precise. No ambiguity or confusion could arise if jewellers made use
of the scientific names for the species, but most of them are unknown
or at least unfamiliar to those unversed in mineralogy, and to banish
old-established names is undesirable, even if the task were not
hopeless. The name selected for a gem-stone may have a very important
bearing on its fortunes. When the love-sick Juliet queried ‘What’s
in a name?’ her mind was wandering far from jewels; for them a name
is everything. The beautiful red stones that accompany the diamond
in South Africa were almost a drug in the market under their proper
title—garnet, but command a ready sale under the misnomer ‘Cape-ruby.’
To many minds there is a subtle satisfaction in the possession of a
stone which is assumed to be a sort of ruby that would be destroyed by
the knowledge that the stone really belonged to the cinderella species
of gem-stones—the despised garnet. For similar reasons it was deemed
advisable to offer the lustrous green garnet found some thirty and odd
years ago in the Ural Mountains as ‘olivine,’ not a happy choice since
their colour is grass- rather than olive-green, apart from the fact
that the term is in general use in science for the species known in
jewellery as peridot.

The names employed in jewellery are largely based upon the colour, the
least reliable from a determinative point of view of all the physical
characters of gem-stones. Qualifying terms are employed to distinguish
stones of obviously different hardness. ‘Oriental’ distinguishes
varieties of corundum, but does not imply that they necessarily came
from the East; the finest gem-stones originally reached Europe by that
road, and the hardest coloured stones consequently received that term
of distinction.

Nearly all red stones are grouped under the name ruby, which is derived
from a Latin word, _ruber_, meaning red, or under other names adapted
from it, such as rubellite, rubicelle. It is properly applied to red
corundum; ‘balas’ ruby is spinel, which is associated with the true
ruby at the Burma mines and is similar in appearance to it when cut,
and ‘Cape’ ruby, is, as has been stated above, a garnet from South
Africa. Rubellite is the lovely rose-pink tourmaline, fine examples of
which have recently been discovered in California, and rubicelle is a
less pronouncedly red spinel. Sapphire is by far the oldest and one
of the most interesting of the words used in the language of jewels.
It occurs in Hebrew and Persian, ancient tongues, and means blue.
It was apparently employed for lapis lazuli or similar substance,
but was transferred to the blue corundum upon the discovery of this
splendid stone. Oblivious of the real meaning of the word, jewellers
apply it in a quasi-generic sense to all the varieties of corundum
with the exception of the red ruby, and give vent to such incongruous
expressions as ‘white sapphire,’ ‘yellow sapphire’; it is true such
stones often contain traces of blue colour, but that is not the reason
of the terms. ‘Brazilian’ sapphire is blue tourmaline, a somewhat rare
tint for this species. The curious history of the word topaz will be
found below in the chapter dealing with the species of that name. It
has always denoted a yellow stone, and at the present day is applied
by jewellers indiscriminately to the true topaz and citrine, the
yellow quartz, the former, however, being sometimes distinguished by
the prefix ‘Brazilian.’ ‘Oriental’ topaz is corundum, and ‘occidental’
topaz is a term occasionally employed for the yellow quartz. Emerald,
which means green, was first used for chrysocolla, an opaque greenish
stone (p. 288), but was afterwards applied to the priceless green
variety of beryl, for which it is still retained. ‘Oriental’ emerald
is corundum, ‘Brazilian’ emerald in the eighteenth century was a
common term for the green tourmaline recently introduced to Europe,
and ‘Uralian’ emerald has been tentatively suggested for the green
garnet more usually known as ‘olivine.’ Amethyst is properly the violet
quartz, but with the prefix ‘oriental’ it is also applied to violet
corundum, though some jewellers use it for the brilliant quartz, with
purple and white sectors, from Siberia. Almandine, which is derived
from the name of an Eastern mart for precious stones, has come to
signify a stone of columbine-red hue, principally garnet, but with
suitable qualification corundum and spinel also.

The nomenclature of jewellery tends to suggest relations between the
gem-stones for which there is no real foundation, and to obscure
the essential identity, except from the point of view of colour, of
sapphire and ruby, emerald and aquamarine, cairngorm and amethyst.

CHAPTER XIV

MANUFACTURED STONES

The initial step in the examination of a crystallized substance is
to determine its physical characters and to resolve it by chemical
analysis into its component elements; the final, and by far the
hardest, step is to build it up or synthetically prepare it from its
constituents. Unknown to the world at large, work of the latter kind
has long been going on within the walls of laboratories, and as the
advance in knowledge placed in the hands of experimenters weapons more
and more comparable with those wielded by nature, their efforts have
been increasingly successful. So stupendous, however, are the powers
of nature that the possibility of reproducing, by human agency, the
treasured stones which are extracted from the earth in various parts
of the globe at the cost of infinite toil and labour has always been
derided by those ignorant of what had already been accomplished. Great,
therefore, was the consternation and the turmoil when concrete evidence
that could not be gainsaid showed that man’s restless efforts to
bridle nature to his will were not in vain, and congresses of all the
high-priests of jewellery were hastily convened to ban such unrighteous
products, with what ultimate success remains to be seen.

Crystallization may be caused in four different ways, of which the
second alone has as yet yielded stones large enough to be cut—

1. By the separation of the substance from a saturated solution. In
nature the solvent may not be merely hot water, or water charged with
an acid, but molten rock, and the temperature and the pressure may be
excessively high.

2. By the solidification of the liquefied substance upon cooling. Ice
is a familiar example of this type.

3. By the sublimation of the vapour of the substance, which means the
direct passage from the vapour to the solid state without traversing
the usually intervening liquid state. It is usually the most difficult
of attainment of the four methods; the most familiar instance is snow.

4. By the precipitation of the substance from a solution when set free
by chemical action.

Other things being equal, the simpler the composition the greater is
the ease with which a substance may be expected to be formed; for,
instead of one complex substance, two or more different substances
may evolve, unless the conditions are nicely arranged. Attempts,
for instance, to produce beryl might result instead in a mixture of
chrysoberyl, phenakite, and quartz.

By far the simplest in composition of all the precious stones is
diamond, which is pure crystallized carbon; but its manufacture is
attended by well-nigh insuperable difficulties. If carbon be heated
in air, it burns at a temperature well below its melting point;
moreover, unless an enormously high pressure is simultaneously applied,
the product is the other form of crystallized carbon, namely, the
comparatively worthless graphite. Moissan’s interesting course of
experiments were in some degree successful, but the tiny diamonds were
worthless as jewels, and the expense involved in their manufacture was
out of all proportion to any possible commercial value they might have.

Next to diamond the simplest substances among precious stones are
quartz (crystallized silica) and corundum (crystallized alumina). The
crystallization of silica has been effected in several ways, but the
value in jewellery of quartz, even of the violet variety, amethyst, is
not such as to warrant its manufacture on a commercial scale. Corundum,
on the other hand, is held in high esteem; rubies and sapphires, of
good colour and free from flaws, have always commanded good prices. The
question of their production by artificial means has therefore more
than academic interest.

Ever since the year 1837, when Gaudin produced a few tiny flakes,
French experimenters have steadily prosecuted their researches in the
crystallization of corundum. Frémy and Feil, in 1877, were the first
to meet with much success. A portion of one of their crucibles lined
with glistening ruby flakes is exhibited in the British Museum (Natural
History).

[Illustration: FIG. 53.—Verneuil’s Inverted Blowpipe.]

In 1885 the jewellery market was completely taken by surprise by the
appearance of red stones, emanating, so it is alleged, from Geneva;
having the physical characters of genuine rubies, they were accepted
as, and commanded the prices of, the natural stones. It was eventually
discovered that they had resulted from the fusion of a number of
fragments of natural rubies in the oxy-hydrogen flame. The original
colour was driven off at that high temperature, but was revived by the
previous addition of a little bichromate of potassium. Owing to the
inequalities of growth, the cracks due to rapid cooling, the inclusion
of air-bubbles, often so numerous as to cause a cloudy appearance,
and, above all, the unnatural colour, these reconstructed stones, as
they are termed, were far from satisfactory, but yet they marked such
an advance on anything that had been accomplished before that for some
time no suspicion was aroused as to their being other than natural
stones.

A notable advance in the synthesis of corundum, particularly of ruby,
was made in 1904, when Verneuil, who had served his apprenticeship
to science under the guidance of Frémy, invented his ingenious
inverted form of blowpipe (Fig. 53), which enabled him to overcome the
difficulties that had baffled earlier investigators, and to manufacture
rubies vying in appearance after cutting with the best of nature’s
productions. The blowpipe consisted of two tubes, of which the upper,
_E_, wide above, was constricted below, and passing down the centre of
the lower, _F_, terminated just above the orifice of the latter in
a fine nozzle. Oxygen was admitted at _C_ through the plate covering
the upper end of the tube, _E_. A rod, which passed through a rubber
collar in the same plate, supported inside the tube, _E_, a vessel,
_D_, and at the upper end terminated in a small plate, on which was
fixed a disc, _B_. The hammer, _A_, when lifted by the action of an
electromagnet and released, fell by gravity and struck the disc. The
latter could be turned about a horizontal axis placed eccentrically,
so that the height through which the hammer fell and the consequent
force of the blow could be regulated. The rubber collar, which was
perfectly gas-tight, held the rod securely, but allowed the shocks to
be transmitted to the vessel, _D_, an arrangement of guides maintaining
the slight motion of the vessel strictly vertical. This vessel, which
carried the alumina powder used in the manufacture of the stone, had as
its base a cylindrical sieve of fine mesh. The succession of rapid taps
of the hammer caused a regular feed of powder down the tube, the amount
being regulated by varying the height through which the hammer fell.
Hydrogen or coal-gas was admitted at _G_ into the outer tube, _F_, and
in the usual way met the oxygen just above the orifice, _L_. To exclude
irregular draughts, the flame was surrounded by a screen, _M_, which
was provided with a mica window, and a water-jacket, _K_, protected the
upper part of the apparatus from excessive heating.

[Illustration: FIG. 54.—‘Boule,’ or Pear-shaped Drop.]

The alumina was precipitated from a solution of pure ammonia—alum,
(NH_{4})_{2}SO_{4}.Al_{2}(SO_{4})_{3}.24H_{2}O, in distilled water
by the addition of pure ammonia, sufficient chrome-alum also being
dissolved with the ammonia-alum to furnish about 2½ per cent. of
chromic oxide in the resulting stone. The powder, carefully prepared
and purified, was placed, as has been stated above, in the vessel,
_D_, and on reaching the flame at the orifice it melted, and fell
as a liquid drop, _N_, upon the pedestal, _P_, which was formed of
previously fused alumina. This pedestal was attached by a platinum
sleeve to an iron rod, _Q_, which was provided with the necessary screw
adjustments, _R_ and _S_, for centring and lowering it as the drop
grew in size. Great care was exercised to free the powder from any
trace of potassium, which, if present, imparted a brownish tinge to
the stone. The pressure of the oxygen, low initially both to prevent
the pedestal from melting, and to keep the area of the drop in contact
with the pedestal as small as possible, because otherwise flaws tended
to start on cooling, was gradually increased until the flame reached
the critical temperature which kept the top of the drop melted, but
not boiling. The supply of powder was at the same time carefully
proportioned to the pressure. The pedestal, _P_, was from time to time
lowered, and the drop grew in the shape of a pear (Fig. 54), the apex
of which was downwards and adhered to the pedestal by a narrow stalk.
As soon as the drop reached the maximum size possible with the size of
the flame, the gases were sharply and simultaneously cut off. After ten
minutes or so the drop was lowered from the chamber, _M_, by the screw,
_S_, and when quite cold was removed from the pedestal.

Very few changes have been made in the method when adapted to
commercial use. Coal-gas has, however, entirely replaced the
costly hydrogen, and the hammer is operated by a cam instead of an
electromagnet, while, as may be seen from the view of a gem-stone
factory (Plate XIV), a number of blowpipes are placed in line so that
their cams are worked by the same shaft, _a_. The fire-clay screen,
_b_, surrounding the flame is for convenience of removal divided into
halves longitudinally, and a small hole is left in front for viewing
the stone during growth, a red glass screen, _c_, being provided in
front to protect the eyes from the intense glare. Half the fire-clay
screen of the blowpipe in the centre of the Plate has been removed to
show the arrangement of the interior. The centring and the raising
and lowering apparatus, _d_, have been modified. The process is so
simple that one man can attend to a dozen or so of these machines, and
it takes only one hour to grow a drop large enough to be cut into a
ten-carat stone.

[Illustration: _PLATE XIV_

BLOWPIPE USED FOR THE MANUFACTURE OF RUBIES AND SAPPHIRES]

The drops, unless the finished stone is required to have a similar
pear shape, are divided longitudinally through the central core into
halves, which in both shape and orientation are admirably suited to the
purposes of cutting; as a general rule, the drop splits during cooling
into the desired direction of its own accord.

[Illustration: FIG. 55.—Bubbles and Curved Striæ in Manufactured Ruby.]

Each drop is a single crystalline individual, and not, as might have
been anticipated, an alumina glass or an irregular aggregation of
crystalline fragments, and, if the drop has cooled properly, the
crystallographic axis is parallel to the core of the pear. The cut
stone will therefore have not only the density and hardness, but also
all the optical characters—refractivity, double refraction, dichroism,
etc.—pertaining to the natural species, and will obey precisely the
same tests with the refractometer and the dichroscope. Were it not for
certain imperfections it would be impossible to distinguish between the
stones formed in Nature’s vast workshop and those produced within the
confines of a laboratory. The artificial stones, however, are rarely,
if ever, free from minute air-bubbles (Fig. 55), which can easily be
seen with an ordinary lens. Their spherical shape differentiates them
from the plane-sided cavities not infrequently visible in a natural
stone (Fig. 56). Moreover, the colouring matter varies slightly, but
imperceptibly, in successive shells, and consequently in the finished
stone a careful eye can discern the curved striations (Fig. 55)
corresponding in shape to the original shell. In a natural stone, on
the other hand, although zones of different colours or varying shades
are not uncommon, the resulting striations are straight (Fig. 56),
corresponding to the plane faces of the original crystal form. By
sacrificing material it might be possible to cut a small stone free
from bubbles, but the curved striations would always be present to
betray its origin.

[Illustration: FIG. 56.—Markings in Natural Ruby.]

The success that attended the manufacture of ruby encouraged efforts to
impart other tints to crystallized alumina. By reducing the percentage
amount of chromic oxide, pink stones were turned out, in colour not
unlike those Brazilian topazes, the original hue of which has been
altered by the application of heat. These artificial stones have
therefore been called ‘scientific topaz’; of course, quite wrongly,
since topaz, which is properly a fluo-silicate of aluminium, is quite a
different substance.

Early attempts made to obtain the exquisite blue tint of the true
sapphire were frustrated by an unexpected difficulty. The colouring
matter, cobalt oxide, was not diffused evenly through the drop, but
was huddled together in splotches, and it was found necessary to add a
considerable amount of magnesia as a flux before a uniform distribution
of colour could be secured. It was then discovered that, despite the
colour, the stones had the physical characters, not of sapphire, but
of the species closely allied to it, namely, spinel, aluminate of
magnesium. By an unsurpassable effort of nomenclature these blue stones
were given the extraordinary name of ‘Hope sapphire,’ from fanciful
analogy with the famous blue diamond which was once the pride of the
Hope collection. A blue spinel is occasionally found in nature, but the
actual tint is somewhat different. These manufactured stones have the
disadvantage of turning purple in artificial light. By substituting
lime for magnesia as a flux, Paris, a pupil of Verneuil’s, produced
blue stones which were not affected to the same extent. The difficulty
was at length overcome at the close of 1909, when Verneuil, by
employing as tinctorial agents 0·5 per cent. of titanium oxide and 1·5
per cent. of magnetic iron oxide, succeeded in producing blue corundum;
it, however, had not quite the tint of sapphire. Stones subsequently
manufactured, which were better in colour, contained about 0·12 per
cent. of titanium oxide, but no iron at all.

By the addition to the alumina of a little nickel oxide and vanadium
oxide respectively, yellow and yellowish green corundums have been
obtained. The latter have in artificial light a distinctly reddish
hue, and have therefore been termed ‘scientific alexandrite’; of
course, quite incorrectly, since the true alexandrite is a variety of
chrysoberyl, aluminate of beryllium, a very different substance.

If no colouring matter at all be added and the alum be free from
potash, colourless stones or white sapphires are formed, which pass
under the name ‘scientific brilliant.’ It is scarcely necessary to
remark that they are quite distinct from the true brilliant, diamond.

The high prices commanded by emeralds, and the comparative success that
attended the reconstruction of ruby from fragments of natural stones,
suggested that equal success might follow from a similar process with
powdered beryl, chromic oxide being used as the colouring agent. The
resulting stones are, indeed, a fair imitation, being even provided
with flaws, but they are a beryl glass with lower specific gravity and
refractivity than the true beryl, and are wrongly termed ‘scientific
emerald.’ Moreover, recently most of the stones so named on the market
are merely green paste.

It is unfortunate that the real success which has been achieved in the
manufacture of ruby and sapphire should be obscured by the ill-founded
claims tacitly asserted in other cases.

At the time the manufactured ruby was a novelty it fetched as much as
£6 a carat, but as soon as it was discovered that it could easily
be differentiated from the natural stone, a collapse took place, and
the price fell abruptly to 30s., and eventually to 5s. and even 1s.
a carat. The sapphires run slightly higher, from 2s. to 7s. a carat.
The prices of the natural stones, which at first had fallen, have now
risen to almost their former level. The extreme disparity at present
obtaining between the prices of the artificial and the natural ruby
renders the fraudulent substitution of the one for the other a great
temptation, and it behoves purchasers to beware where and from whom
they buy, and to be suspicious of apparently remarkable bargains,
especially at places like Colombo and Singapore where tourists abound.
It is no secret that some thousands of carats of manufactured rubies
are shipped annually to the East. _Caveat emptor._

CHAPTER XV

IMITATION STONES

The beryl glass mentioned in the previous chapter marks the transition
stage between manufactured stones which in all essential characters
are identical with those found in nature, and artificial stones which
resemble the corresponding natural stone in outward appearance only. In
a sense both sorts may be styled artificial, but it would be misleading
to confound them under the same appellation.

Common paste,[6] which is met with in drapery goods and cheap ornaments
in general—hat-pins, buckles, and so forth—is composed of ordinary
crown-glass or flint-glass, the refractive indices being about 1·53 and
1·63 respectively. The finest quality, which is used for imitations of
brilliants, is called ‘strass.’ It is a dense lead flint-glass of high
refraction and strong colour-dispersion, consisting of 38·2 per cent.
of silica, 53·3 red lead (oxide of lead), and 7·8 potassium carbonate,
with small quantities of soda, alumina, and other substances. How
admirable these imitations may be, a study of the windows of a shop
devoted to such things will show. Unfortunately the addition of
lead, which is necessary for imparting the requisite refraction and
‘fire’ to the strass, renders the stones exceedingly soft. All glass
yields to the file, but strass stones are scratched even by ordinary
window-glass. If worn in such a way that they are rubbed, they
speedily lose the brilliance of their polish, and, moreover, they are
susceptible to attack by the sulphurous fumes present in the smoky air
of towns, and turn after a time a dirty brown in hue. When coloured
stones are to be imitated, small quantities of a suitable metallic
oxide are fused with the glass; cobalt gives rise to a royal-blue tint,
chromium a ruby red, and manganese a violet. Common paste is not highly
refractive enough to give satisfactory results when cut as a brilliant,
and the bases are therefore often coated with quicksilver, or, in the
case of old jewellery, covered with foil in the setting, in order to
secure more complete reflection from the interior. The fashioning of
these imitation stones is easy and cheap. Being moulded, they do not
require cutting, and the polishing of the facets thus formed is soon
done on account of the softness of the stones.

A test with a file readily differentiates paste stones from the natural
stones they pretend to be. Being necessarily singly refractive, they
are, of course, lacking in dichroism, and their refractivity seldom
accords even approximately with that of the corresponding natural stone.

In order to meet the test for hardness the doublet was devised. Such
a stone is composed of two parts—the crown consisting of colourless
quartz or other inexpensive real and hard stone, and the base being
made up of coloured glass. When the imitation, say of a sapphire, is
intended to be more exact, the crown is made of a real sapphire, but
one deficient in colour, the requisite tint being obtained from the
paste forming the under part of the doublet. In case the base should
also be tested for hardness the triplet has been devised. In this the
base is made of a real stone also, and the coloured paste is confined
to the girdle section, where it is hidden by the setting. Sapphires and
emeralds of indifferent colour are sometimes slit across the girdle;
the interior surfaces are polished, and colouring matter is introduced
with the cement, generally Canada balsam, which is used to re-unite the
two portions of the stone together. All such imitations may be detected
by placing the stone in oil, when the surfaces separating the portions
of the composite stone will be visible, or the binding cement may be
dissolved by immersing the stone, if unmounted, in boiling water, or in
alcohol or chloroform, when the stone will fall to pieces.

The glass imitations of pearls, which have become very common in recent
years, may, apart from their inferior iridescence, be detected by
their greater hardness, or by the apparent doubling of, say, a spot of
ink placed on the surface, owing to reflection from the inner surface
of the glass shell. They are made of small hollow spheres formed by
blowing. Next to the glass comes a lining of parchment size, and next
the under lining, which is the most important part of the imitation,
consisting of a preparation of fish scales called _Essence d’Orient_,
When the lining is dry, the globe is filled with hot wax to impart the
necessary solidity. In cheap imitations the glass balls are not lined
at all, but merely heated with hydrochloric acid to give an iridescence
to the surface; sometimes they are coated with wax, which can be
scraped off with a knife.

PART II—SECTION A

PRECIOUS STONES

CHAPTER XVI

DIAMOND

Diamond has held pride of place as chief of precious stones ever
since the discovery of the form of cutting known as the ‘brilliant’
revealed to full perfection its amazing qualities; and justly so,
since it combines in itself extreme hardness, high refraction, large
colour-dispersion, and brilliant lustre. A rough diamond, especially
from river gravels, has often a peculiar greasy appearance, and is
no more attractive to the eye than a piece of washing-soda. It is
therefore easy to understand why the Persians in the thirteenth century
placed the pearl, ruby, emerald, and even peridot before it, and
writers in the Middle Ages frequently esteemed it below emerald and
ruby. The Indian lapidaries, who were the first to realize that diamond
could be ground with its own powder, discovered what a wonderful
difference the removal of the skin makes in the appearance of a stone.
They, however, made no attempt to shape a stone, but merely polished
the natural facets, and only added numerous small facets when they
wished to conceal flaws or other imperfections; indeed, the famous
traveller, Tavernier, from whom most of our knowledge of early mining
in India is obtained, invariably found that a stone covered with many
facets was badly flawed. The full radiant beauty of a diamond comes to
light only when it is cut in brilliant form.

Of all precious stones diamond has the simplest composition; it is
merely crystallized carbon, another form of which is the humble and
useful graphite, commonly known as ‘black-lead.’ Surely nature has
surpassed all her marvellous efforts in producing from the same element
substances with such divergent characters as the hard, brilliant, and
transparent diamond and the soft, dull, and opaque graphite. It is,
however, impossible to draw any sharp dividing line between the two;
soft diamond passes insensibly into hard graphite, and vice versa.
Boart, or bort, as it is sometimes written, is composed of minute
crystals of diamond arranged haphazardly; it possesses no cleavage,
its hardness is greater than that of the crystals, and its colour is
greyish to blackish. Carbon, carbonado, or black diamond, which is
composed of still more minute crystals, is black and opaque, and is
perceptibly harder than the crystals. It passes into graphite, which
varies in hardness, and may have any density between 2·O and 3·O.
Jewellers apply the term boart to crystals or fragments which are of no
service as gems; such pieces are crushed to powder and used for cutting
and polishing purposes.

Diamonds, when absolutely limpid and free from flaws, are said to be
of the ‘first water,’ and are most prized when devoid of any tinge of
colour except perhaps bluish (Plate I, Fig. 1). Stones with a slight
tinge of yellow are termed ‘off-coloured,’ and are far less valuable.
Those of a canary-yellow colour (Plate I, Fig. 3), however, belong to a
different category, and have a decided attractiveness. Greenish stones
also are common, though it is rare to come across one with a really
good shade of that colour. Brown stones, especially in South Africa,
are not uncommon. Pink stones are less common, and ruby-red and blue
stones are rare. Those of the last-named colour have usually what is
known as a ‘steely’ shade, _i.e._ they are tinged with green; stones of
a sapphire blue are very seldom met with, and such command high prices.

[Illustration: FIGS. 57—59.—Diamond Crystals.]

Diamond crystallizes (Figs. 57—59 and Plate I, Fig. 2) in octahedra
with brilliant, smooth faces, and occasionally in cubes with rough
pitted faces; sometimes three or six faces take the place of each
octahedron face, and the stone is almost spherical in shape. The
surfaces of the crystals are often marked with equilateral triangles,
which are supposed to represent the effects of incipient combustion.
Twinned crystals, in which the two individuals may be connected by a
single plane or may be interpenetrating, a star shape often resulting
in the latter case, are common; sometimes, if of the octahedron type,
they are beautifully symmetrical. The rounded crystals are frequently
covered with a peculiar gum-like skin which is somewhat less hard than
the crystal itself. A large South African stone, weighing 27 grams (130
carats) and octahedral in shape, which was the gift of John Ruskin,
and named by him the ‘Colenso’ after the first bishop of Natal, is
exhibited in the British Museum (Natural History); its appearance is,
however, marred by its distinctly ‘off-coloured’ tint.

The refraction of diamond is single, but local double refraction is
common, indicating a state of strain which can often be traced to an
included drop of liquid carbonic acid; so great is the strain that many
a fine stone has burst to fragments on being removed from the ground
in which it has lain. The refractive index for the yellow light of a
sodium flame is 2·4175, and the slight variation from this mean value
that has been observed, amounting only to 0·0001, testifies to the
purity of the composition. The colour-dispersion is large, being as
much as 0·044, in which respect it surpasses all colourless stones,
but is exceeded by sphene and the green garnet from the Urals (cf. p.
217). The lustre of diamond, when polished, is so characteristic as to
be termed adamantine, and is due to the combination of high refraction
and extreme hardness. Diamond is translucent to the X (Röntgen) rays;
it phosphoresces under the action of radium, and of a high-tension
electric current when placed in a vacuum tube, and sometimes even when
exposed to strong sunlight. Some diamonds fluoresce in sunlight,
turning milky, and a few even emit light when rubbed. Crookes found
that a diamond buried in radium bromide for a year had acquired a
lovely blue tint, which was not affected even by heating to redness.
The specific gravity is likewise constant, being 3·521, with a possible
variation from that mean value of 0·005; but a greater range, as might
be expected, is found in the impure boart.

Diamond is by far the hardest substance in nature, being marked 10 in
Mohs’s scale of hardness, but it varies in itself; stones from Borneo
and New South Wales are so perceptibly harder than those usually in
the lapidaries’ hands, that they can be cut only with their own and
not ordinary diamond powder, and some difficulty was experienced in
cutting them when they first came into the market. It is interesting
to note that the metal tantalum, the isolation of which in commercial
amount constituted one of the triumphs of chemistry of recent years,
has about the same hardness as diamond. Despite its extreme hardness
diamond readily cleaves under a heavy blow in planes parallel to the
faces of the regular octahedron, a property utilized for shaping the
stone previous to cutting it. The fallacious, but not unnatural,
idea was prevalent up to quite modern times that a diamond would,
even if placed on an anvil, resist a blow from a hammer: who knows
how many fine stones have succumbed to this illusory test? The fact
that diamond could be split was known to Indian lapidaries at the
time of Tavernier’s visit, and it would appear from De Boodt that in
the sixteenth century the cleavability of diamond was not unknown in
Europe, but it was not credited at the time and was soon forgotten.
Early last century Wollaston, a famous chemist and mineralogist,
rediscovered the property, and, so it is said, used his knowledge to
some profit by purchasing large stones, which because of their awkward
shape or the presence of flaws in the interior were rejected by the
lapidaries, and selling them back again after cleaving them to suitable
forms.

It has already been remarked (p. 79) that the interval in hardness
between diamond and corundum, which comes next to it in Mohs’s scale,
is enormously greater than that between corundum and the softest of
minerals. Diamond can therefore be cut only with the aid of its own
powder, and the cutting of diamond is therefore differentiated from
that of other stones, the precious-stone trade being to a large extent
divided into two distinct groups, namely, dealers in diamonds, and
dealers in all other gem-stones.

The name of the species is derived from the popular form, _adiamentem_,
of the Latin _adamantem_, itself the alliterative form of the Greek
ἀδάμας, meaning the unconquerable, in allusion not merely to the
great hardness but also to the mistaken idea already mentioned. Boart
probably comes from the Old-French _bord_ or _bort_, bastard.

At the present day diamonds are usually cut as brilliants, though
the contour of the girdle may be circular, oval, or drop-shaped to
suit the particular purpose for which the stone is required, or to
keep the weight as great as possible. Small stones for bordering a
large coloured stone may also be cut as roses or points. A perfect
brilliant has 58 facets, but small stones may have not more than 44,
and exceptionally large stones may with advantage have many more; for
instance, on the largest stone cut from the Cullinan diamond there are
no fewer than 74 facets.

The description of the properties of diamond would not be complete
without a reference to the other valuable, if utilitarian, purposes
to which it is put. Without its aid much of modern engineering work
and mining operations would be impossible except at the cost of almost
prohibitive expenditure of time and money.

Boring through solid rock has been greatly facilitated by the use of
the diamond drill. For this purpose carbonado or black diamond is more
serviceable than single crystals, and the price of the former has
consequently advanced from a nominal figure up to £3 to £12 a carat.
The actual working part of the drill consists of a cast-steel ring.
The crown of it has a number of small depressions at regular intervals
into which the carbonados are embedded. On revolution of the drill an
annular ring is cut, leaving a solid core which can be drawn to the
surface. For cooling the drill and for washing away the detritus water
is pumped through to the working face. The duration of the carbonados
depends on the nature of the rock and the skill of the operator. The
most troublesome rock is a sandstone or one with sharp differences in
hardness, because the carbonados are liable to be torn out of their
setting. An experienced operator can tell by the feel of the drill the
nature of the rock at the working face, and by varying the pressure can
mitigate the risk of damage to the drill.

The tenacity of diamond renders it most suitable for wire-drawing. The
tungsten filaments used in many of the latest forms of incandescent
electric lamps are prepared in this manner.

Diamond powder is used for cutting and turning the hardened steel
employed in modern armaments and for other more peaceful purposes.

Although nearly all the gem-stones scratch glass, diamond alone can be
satisfactorily employed to cut it along a definite edge. Any flake at
random will not be suitable, because it will tear the glass and form a
jagged edge. The best results are given by the junction of two edges
which do not meet in too obtuse an angle; two edges of the rhombic
dodecahedron meet the requirements admirably. The stones used by the
glaziers are minute in size, being not much larger than a pin’s head,
and thirty of them on an average go to the carat. They are set in
copper or brass. Some little skill is needed to obtain the best results.

The value of a diamond has always been determined largely by the size
of the stone, the old rule being that the rate per carat should be
multiplied by the square of the weight in carats; thus, if the rate be
£10, the cost of a two-carat stone is four times this sum, or £40, of
a three-carat stone £90, and so on. For a century, from 1750 to 1850,
the rate remained almost constant at £4 for rough, £6 for rose-cut,
and £8 for brilliant-cut diamonds. Since the latter date, owing to
the increase in the supply of gold, the growth of the spending power
of the world, and the gradual falling off in the productiveness of
the Brazilian fields, the rate steadily increased about 10 per cent.
each year, until in 1865 the rate for brilliants was £18. The rise was
checked by the discovery of the South African mines; moreover. since
comparatively large stones are plentiful in these mines, the rule
of the increase in the price of a stone by the square of its weight
no longer holds. The rate for the most perfect stones still remains
high, because such are not so common in the South African mines. The
classification[7] adopted by the syndicate of London diamond merchants
who place upon the market the output of the De Beers group of mines is
as follows:—(_a_) Blue-white, (_b_) white, (_c_) silvery Cape, (_d_)
fine Cape, (_e_) Cape, (_f_) fine bywater, (_g_) bywater, (_h_) fine
light brown, (_i_) light brown, (_j_) brown, (_k_) dark brown. Bywaters
or byes are stones tinged with yellow.

The rate per carat for cut stones in the blue-white and the bywater
groups is:—

BLUE-WHITE. BYWATER.
5-carat stone £40-60 £20-25
1 „ 30-40 10-15
½ „ 20-25 8-12
¼ „ 15-18 6-10
Mêlée 12-15 5-8

Mêlée are stones smaller than a quarter of a carat. It will be noticed
that the prices depart largely from the old rule; thus taking the rate
for a carat blue-white stone, the price of a five-carat stone should
be from £150-200 a carat, and for a quarter-carat stone only £7, 10s.
to £10 a carat. There happens to be at the time of writing very little
demand for five-carat stones. Of course, the prices given are subject
to constant fluctuation depending upon the supply and demand, and the
whims of fashion.

CHAPTER XVII

OCCURRENCE OF DIAMOND

The whole of the diamonds known in ancient times were obtained from
the so-called Golconda mines in India. Golconda itself, now a deserted
fortress near Hyderabad, was merely the mart where the diamonds were
bought and sold. The diamond-bearing district actually spread over a
wide area on the eastern side of the Deccan, extending from the Pinner
River in the Madras Presidency northwards to the Rivers Son and Khan,
tributaries of the Ganges, in Bundelkhand. The richest mines, where
the large historical stones were found, are in the south, mostly
near the Kistna River. The diamonds were discovered in sandstone,
or conglomerate, or the sands and gravels of river-beds. The mines
were visited in the middle of the seventeenth century by the French
traveller and jeweller, Tavernier, when travelling on a commission
for Louis XIV, and he afterwards published a careful description of
them and of the method of working them. The mines seem to have been
exhausted in the seventeenth century; at any rate, the prospecting,
which has been spasmodically carried on during the last two centuries,
has proved almost abortive. With the exception of the Koh-i-nor, all
the large Indian diamonds were probably discovered not long before
Tavernier’s visit. The diamonds known to Pliny, and in his time, were
quite small, and it is doubtful if any stones of considerable size came
to light before A.D. 1000.

India enjoyed the monopoly of supplying the world’s demand for diamonds
up to the discovery, in 1725, of the precious stone in Brazil. Small
stones were detected by the miners in the gold washings at Tejuco,
about eighty miles (129 km.) from Rio de Janeiro, in the Serro do Frio
district of the State of Minas Geraes. The discovery naturally caused
great excitement. So many diamonds were found that in 1727 something
like a slump took place in their value. In order to keep up prices, the
Dutch merchants, who mainly controlled the Indian output, asserted that
the diamonds had not been found in Brazil at all, but were inferior
Indian stones shipped to Brazil from Goa. The tables were neatly turned
when diamonds were actually shipped from Brazil to Goa, and exported
thence to Europe as Indian stones. This course and the continuous
development of the diamond district in Brazil rendered it impossible
to hoodwink the world indefinitely. The drop in prices was, however,
stayed by the action of the Portuguese government, who exacted such
heavy duties and imposed such onerous conditions that finally no one
would undertake to work the mines. Accordingly, in 1772 diamond-mining
was declared a royal monopoly in Brazil, and such it remained until
the severance of Brazil from Portugal in 1834, when private mining was
permitted by the new government subject to the payment of reasonable
royalties. The industry was enormously stimulated by the discovery, in
1844, of the remarkably rich fields in the State of Bahia, especially
at Serra da Cincorá, where carbonado, or black diamond, first came to
light, but after a few years, owing to the difficulties of supplying
labour, the unhealthiness of the climate, and the high cost of living,
the yield fell off and gradually declined, until the importance of the
fields was finally eclipsed by the rise of the South African mines.
The Brazilian mines have proved very productive, but chiefly in small
diamonds, stones above a carat in weight being few in comparison. The
largest stone, to which the name, the Star of the South, was applied,
weighed in the rough 254½ carats; it was discovered at the Bagagem
mines in 1853. The quality of the diamonds is good, many of them having
the highly-prized bluish-white colour. The principal diamond-bearing
districts of Brazil centre at Diamantina, as Tejuco was re-named
after the discovery of diamonds, Grão Magor, and Bagagem in the State
of Minas Geraes, at Diamantina in the State of Bahia, and at Goyãz
and Matto Grosso in the States of the same names. The diamonds occur
chiefly in _cascalho_, a gravel, containing large masses of quartz
and small particles of gold, which is supposed to be derived from a
quartzose variety of micaceous slate known as itacolumite. The mines
are now to some extent being worked by systematic dredging of the
river-beds.

Early in 1867 the children of a Boer farmer, Daniel Jacobs, who dwelt
near Hopetown on the banks of the Orange River, picked up in the course
of play near the river a white pebble, which was destined not only to
mark the commencement of a new epoch in the record of diamond mines,
but to change the whole course of the history of South Africa. This
pebble attracted the attention of a neighbour, Schalk van Niekerk, who
suspected that it might be of some value, and offered to buy it. Mrs.
Jacobs, however, gave it him, laughingly scouting the idea of accepting
money for a mere pebble. Van Niekerk showed it to a travelling trader,
by name John O’Reilly, who undertook to obtain what he could for it on
condition that they shared the proceeds. Every one he met laughed to
scorn the idea that the stone had any value, and it was once thrown
away and only recovered after some search in a yard, but at length he
showed it to Lorenzo Boyes, the Acting Civil Commissioner at Colesberg,
who, from its extreme hardness, thought it might be diamond and sent
it to the mineralogist, W. Guybon Atherston, of Grahamstown, for
determination. So uncertain was Boyes of its value that he did not even
seal up the envelope containing it, much less register the package.
Atherston found immediately that the long-scorned pebble was really
a fine diamond, weighing 21-3/16 carats, and with O’Reilly’s consent
he submitted it to Sir Philip Wodehouse, Governor at the Cape. The
latter purchased it at once for £500, and dispatched it to be shown
at the Paris Exhibition of that year. It did not, however, attract
much attention; chimerical tales of diamond finds in remote parts of
the world are not unknown. Indeed, for some time only a few small
stones were picked up beside the Orange River, and no one believed in
the existence of any extensive diamond deposit. However, all doubt as
to the advisibility of prospecting the district was settled by the
discovery of the superb diamond, afterwards known as the ‘Star of
South Africa,’ which was picked up in March 1869 by a shepherd boy
on the Zendfontein farm near the Orange River. Van Niekerk, on the
alert for news of further discoveries, at once hurried to the spot and
purchased the stone from the boy for five hundred sheep, ten oxen, and
a horse, which seemed to the boy untold wealth, but was not a tithe of
the £11,200 which Lilienfeld Bros., of Hopetown, gave Van Niekerk.

[Illustration: _PLATE XV_

KIMBERLEY MINE, 1871]

[Illustration: _PLATE XVI_

KIMBERLEY MINE, 1872]

This remarkable discovery attracted immediate attention to the
potentialities of a country which produced diamonds of such a size, and
prospectors began to swarm into the district, gradually spreading up
the Vaal River. For some little time not much success was experienced,
but at length, early in 1870, a rich find was made at Klipdrift,
now known as Barkly West, which was on the banks of the Vaal River
immediately opposite the Mission camp at Pniel. The number of miners
steadily increased until the population on the two sides of the river
included altogether some four or five thousand people, and there was
every appearance of stability in the existing order of things. But a
vast change came over the scene upon the discovery of still richer
mines lying to the south-east and some distance from the river. The
ground was actually situated on the route traversed by parties hurrying
to the Vaal River, but no one dreamed of the wealth that lay under
their feet. The first discovery was made in August 1870 at the farm
Jagersfontein, near Fauresmith in Orange River Colony, by De Klerk,
the intelligent overseer, who noticed in the dry bed of a stream a
number of garnets, and, knowing that they often accompanied diamond,
had the curiosity to investigate the point. He was immediately
rewarded by finding a fine diamond weighing 50 carats. In the following
month diamonds were discovered about twenty miles from Klipdrift
at Dutoitspan on the Dorstfontein farm, and a little later also on
the contiguous farm of Bultfontein; a diamond was actually found in
the mortar used in the homestead of the latter farm. Early in May
1871 diamonds were found about two miles away on De Beers’ farm,
Vooruitzigt, and two months later, in July, a far richer find was made
on the same farm at a spot which was first named Colesberg Kopje, the
initial band of prospectors having come from the town of that name
near the Orange River, but was subsequently known as Kimberley after
the Secretary of State for the Colonies at that time. Soon a large
and prosperous town sprang up close to the mines; it rapidly grew
in size and importance, and to this day remains the centre of the
diamond-mining industry. Subsequent prospecting proved almost blank
until the discovery of the Premier or Wesselton mine on Wesselton
farm, about four miles from Kimberley, in September 1890; it received
the former name after Rhodes, who was Premier of Cape Colony at that
date. No further discovery of any importance was made until, in 1902,
diamonds were found about twenty miles north-west-north of Pretoria in
the Transvaal, at the new Premier mine, now famed as the producer of
the gigantic Cullinan diamond.

[Illustration: _PLATE XVII_

KIMBERLEY MINE, 1874]

[Illustration: _PLATE XVIII_

KIMBERLEY MINE, 1881]

The Kimberley mines were at first known as the ‘dry diggings’ on
account of their arid surroundings in contradistinction to the ‘river
diggings’ by the Vaal. The dearth of water was at first one of the
great difficulties in the way of working the former mines, although
subsequently the accumulation of underground water at lower levels
proved a great obstacle to the working of the mines. The ‘river
diggings’ were of a type similar to that met with in India and Brazil,
the diamonds occurring in a gravelly deposit of limited thickness
beneath which was barren rock, but the Kimberley mines presented a
phenomenon hitherto without precedent in the whole history of diamond
mining. The diamonds were found in a loose surface deposit, which was
easily worked, and for some time the prospectors thought that the
underlying limestone corresponded to the bedrock of the river gravel,
until at length one more curious than his fellows investigated the
yellowish ground underneath, and found to his surprise that it was even
richer than the surface layer. Immediately a rush was made back to the
deserted claims, and the mines were busier than ever. This ‘yellow
ground,’ as it is popularly called, was much decomposed and easy,
therefore, to work and sift. About fifty to sixty feet (15-18 m.) below
the surface, however, it passed into a far harder rock, which from its
colour is known as the ‘blue ground’; this also, to the unexpected
pleasure of the miners, turned out to contain diamonds. Difficulties
arose as each claim, 30 by 30 Dutch feet (about 31 English feet or
9·45 metres square) in area, was worked downwards. In the Kimberley
mine (Plate XVI) access to the various claims was secured by retaining
parallel strips, 15 feet wide, each claim being, therefore, reduced
in width to 22½ feet, to form roadways running from side to side of
the mine in one direction. These, however, soon gave way, not only
because of the falling of the earth composing them, but because they
were undermined and undercut by the owners of the adjacent claims.
By the end of 1872 the last roadway had disappeared, and the mine
presented the appearance of a vast pit. In order to obtain access to
the claims without intruding on those lying between, and to provide for
the hauling of the loads of earth to the surface, an ingenious system
of wire cables in three tiers (Plate XVII) was erected, the lowest
tier being connected to the outermost claims, the second to claims
farther from the edge, and the highest to claims in the centre of the
pit. The mine at that date presented a most remarkable spectacle,
resembling an enormous radiating cobweb, which had a weird charm by
night as the moonlight softly illuminated it, and by day, owing to
the perpetual ring of the flanged wheels of the trucks on the running
wires, twanged like some gigantic æolian harp. This system fulfilled
its purpose admirably until, with increasing depth of the workings,
other serious difficulties arose. Deprived of the support of the hard
blue ground, the walls of the mine tended to collapse, and additional
trouble was caused by the underground water that percolated into the
mine. By the end of 1883 the floor of the Kimberley mine was almost
entirely covered by falls of ‘reef’ (Plate XVIII), as the surrounding
rocks are termed, the depth then being about 400 feet (122 m.). In the
De Beers mine, in spite of the precaution taken to prevent falls of
reef by cutting the walls of the mine back in terraces, falls occurred
continuously in 1884, and by 1887, at a depth of 350 feet (107 m.),
all attempts at open working had to be abandoned. In the Dutoitspan
mine buttresses of blue ground were left, which held back the reef for
some years, but ultimately the mine became unsafe, and in March 1886 a
disastrous fall took place, in which eighteen miners—eight white men
and ten Kafirs—lost their lives. The Bultfontein mine was worked to the
great depth of 500 feet (152 m.), but falls occurred in 1889 and put
an end to open working. In all cases, therefore, the ultimate end was
the same: the floor of the mine became covered with a mass of worthless
reef, which rendered mining from above ground dangerous, and, indeed,
impossible except at prohibitive cost. It was then clearly necessary
to effect access to the diamond-bearing ground by means of shafts sunk
at a sufficient distance from the mine to remove any fear of falls of
reef. For such schemes co-operative working was absolutely essential.
Plate XIX illustrates the desolate character of the Kimberley mine
above ground and the vastness of the yawning pit, which is over 1000
feet (300 m.) in depth.

[Illustration: _PLATE XIX_

KIMBERLEY MINE AT THE PRESENT DAY]

[Illustration: _PLATE XX_

WESSELTON (_open_) MINE]

A certain amount of linking up of claims had already taken place, but,
although many men must have seen that the complete amalgamation of the
interests in each mine was imperative, two men alone had the capacity
to bring their ideas to fruition. C. J. Rhodes was the principal agent
in the formation in April 1880 of the De Beers Mining Company, which
rapidly absorbed the remaining claims in the mine, and was re-formed
in 1887 as the De Beers Consolidated Mining Company. Meantime, Barnett
Isaacs, better known by the cognomen Barnato, which had been adopted by
his

brother Henry when engaged in earning his livelihood in the diamond
fields as an entertainer, had secured the major interests in the
Kimberley mine. Rhodes saw that, for effective working of the two
mines by any system of underground working, they must be under one
management, but to all suggestions of amalgamation Barnato remained
deaf, and at last Rhodes determined to secure control of the Kimberley
mine at all costs. The story of the titanic struggle between these
two men forms one of the epics of finance. Eventually, when shares
in the Kimberley mine had been boomed to an extraordinary height,
and the price of diamonds had fallen as low as 18s. a carat, Barnato
gave way, and in July 1889 the Kimberley mine was absorbed by the De
Beers Company on payment of the enormous sum of £5,338,650. Shortly
afterwards they undertook the working of the Dutoitspan and the
Bultfontein mines, and in January 1896 they acquired the Premier or
Wesselton mine. The interests in the Jagersfontein mine were in 1888
united in the New Jagersfontein Mining and Exploration Company, and
the mine is now worked also by the De Beers Company. Thus, until the
development of the new Premier mine in the Transvaal, the De Beers
Company practically controlled the diamond market. The development
of this last mine was begun so recently, and its size is so vast—the
longest diameter being half a mile—that open-cut working is likely to
continue for some years.

[Illustration: _PLATE XXI_

LOADING THE BLUE GROUND ON THE FLOORS, AND PLOUGHING IT OVER]

[Illustration: _PLATE XXII_

WASHING-MACHINES FOR CONCENTRATING THE BLUE GROUND]

Though varying slightly in details, the methods of working the mines
are identical in principle. From the steeply inclined shaft horizontal
galleries are run diagonally right across the mine, the vertical
interval between successive galleries being 40 feet. From each
gallery side galleries are run at right angles to it and parallel to
the working face. The blue ground is worked systematically backwards
from the working face. The mass is stoped, _i.e._ drilled and broken
from the bottom upwards, until only a thin roof is left. As soon as the
section is worked out and the material removed, the roof is allowed
to fall in, and work is begun on the next section of the same level;
at the same time the first section on the level next below is opened
out. Thus work is simultaneously carried on in several levels, and a
vertical plane would intersect the working faces in a straight line
obliquely inclined to the vertical direction (Fig. 60). When freshly
mined, the blue ground is hard and compact, but it soon disintegrates
under atmospheric influence. Indeed, the yellow ground itself was
merely decomposed blue ground. No immediate attempt is made, therefore,
to retrieve the precious stones. The blue ground is spread on to
the ‘floors’ (Plate XXI), _i.e._ spaces of open veldt which have
been cleared of bushes and inequalities, to the depth of a couple of
feet, and remains there for periods ranging from six months to two
years, depending on the quality of the blue ground and the amount of
rainfall. To hasten the disintegration the blue ground is frequently
ploughed over and occasionally watered, a remarkable introduction of
agricultural methods into mining operations. No elaborate patrolling
or guarding is required, because the diamonds are so sparsely, though
regularly, scattered through the mass that even of the actual workers
in the mines but few have ever seen a stone in the blue ground. When
sufficiently broken up, it is carted to the washing and concentrating
machines, by means of which the diamonds and the heavier constituents
are separated from the lighter material.

[Illustration: FIG. 60.—Vertical Section of Diamond Pipe, showing
Tunnels and Stopes.]

Formerly the diamonds were picked out from the concentrates by means
of the keen eyes of skilled natives; but the process has been vastly
simplified and the risk of theft entirely eliminated by the remarkable
discovery made in 1897 by F. Kirsten, of the De Beers Company, that
of all the heavy constituents of the blue ground diamond alone, with
the exception of an occasional corundum and zircon, which are easily
sorted out afterwards, adheres to grease more readily than to water.
In this ingenious machine, the ‘jigger’ or ‘greaser’ (Plate XXIII) as
it is commonly termed, the concentrates are washed over a series of
galvanized-iron trays, which are covered with a thick coat of grease.
The trays are slightly inclined downwards, and are kept by machinery
in constant sideways motion backwards and forwards. So accurate is the
working of this device that few diamonds succeed in getting beyond
the first tray, and none progress as far as the third, which is added
as an additional precaution. The whole apparatus is securely covered
in so that there is no risk of theft during the operation. The trays
are periodically removed, and the grease is scraped off and boiled to
release the diamonds, the grease itself being used over again on the
trays. This is the first time in the whole course of extraction from
the mines that the diamonds are actually handled. The stones are now
passed on to the sorters, who separate them into parcels according to
their size, shape, and quality.

[Illustration: _PLATE XXIII_

DIAMOND-SORTING MACHINES]

[Illustration: _PLATE XXIV_

KAFIRS PICKING OUT DIAMONDS]

The classification at the mines is first into groups by the shape:
(1) close goods, (2) spotted stones, (3) rejection cleavage, (4)
fine cleavage, (5) light brown cleavage, (6) ordinary and rejection
cleavage, (7) flats, (8) macles, (9) rubbish, (10) boart. Close goods
are whole crystals which contain no flaws and can be cut into single
stones. Spotted stones, as their name suggests, contain spots which
necessitate removal, and cleavage includes stones which are so full of
flaws that they have to be cleaved or split into two or more stones.
Flats are distorted octahedra, and macles are twinned octahedra.
Rubbish is material which can be utilized only for grinding purposes,
and boart consists of round dark stones which are invaluable for
rock-drills. These groups are afterwards graded into the following
subdivisions, depending on increasing depth of yellowish tint: (_a_)
blue-white, (_b_) first Cape, (_c_) second Cape, (_d_) first bye, (_e_)
second bye, (_f_) off-colour, (_g_) light yellow, (_h_) yellow. It is,
however, only the first group that is so minutely subdivided. After
being purchased, the parcels are split up again somewhat differently
for the London market (cf. p. 136), and the dealers re-arrange the
stones according to the purpose for which they are required. Formerly
a syndicate of London merchants took the whole of the produce of the
Kimberley mines at a previously arranged price per carat, but at
the present time the diamonds are sold by certain London firms on
commission.

The products of each mine show differences in either form or colour
which enable an expert readily to recognize their origin. The old
diggings by the Vaal River yielded finer and more colourless stones
than those found in the dry diggings and the mines underlying them. The
South African diamonds, taken as a whole, are always slightly yellowish
or ‘off-coloured’; the mines are, indeed, remarkable for the number of
fine and large, canary-yellow and brown, stones produced. The Kimberley
mine yields a fair percentage of white, and a large number of twinned
and yellow stones. The yield of the De Beers mine comprises mostly
tinted stones—yellow and brown, occasionally silver capes, and very
seldom stones free from colour. The Dutoitspan mine is noted for its
harvest of large yellow diamonds; it also produces fine white cleavage
and small white octahedra. The stones found in the Bultfontein mine are
small and spotted, but, on the other hand, the yield has been unusually
regular. The Premier or Wesselton mine yields a large proportion
of flawless octahedra, but, above all, a large number of beautiful
deep-orange diamonds. Of all the South African mines the Jagersfontein
in the Orange River Colony alone supplies stones of the highly-prized
blue-white colour and steely lustre characteristic of the old Indian
stones. The new Premier mine in the Transvaal is prolific, but mostly
in off-coloured and low-grade stones, the Cullinan diamond being a
remarkable exception.

To illustrate the amazing productiveness of the South African mines,
it may be mentioned that, according to Gardner F. Williams, the
Kimberley group of mines in sixteen years yielded 36 million carats
of diamonds, and the annual output of the Jagersfontein mine averages
about a quarter of a million carats, whereas the total output of the
Brazil mines, for the whole of the long period during which they have
been worked, barely exceeds 13 million carats. The average yield of the
South African mines, however, perceptibly diminishes as the depth of
the mines increases.

The most interesting point connected with the South African diamond
mines, viewed from the scientific standpoint, is the light that they
have thrown on the question of the origin of the diamond, which
previously was an incomprehensible and apparently insoluble problem.
In the older mines, just as at the river diggings by the Vaal, the
stones are found in a gravelly deposit that has resulted from the
disintegration of the rocks through which the adjacent river has
passed, and it is clear that the diamond cannot have been formed _in
situ_ here; it had been suspected, and now there is no doubt, that the
itacolumite rock of Brazil has consolidated round the diamonds which
are scattered through it, and that it cannot be the parent rock. The
occurrence at Kimberley is very different. These mines are funnels
which go downwards to unknown depths; they are more or less oval in
section, becoming narrower with increasing depth, and are evidently the
result of some eruptive agency. The Kimberley mine has been worked to
a depth of nearly 4000 feet (1200 m.), and no signs of a termination
have as yet appeared. The blue ground which fills these ‘pipes,’ as
they are termed, must have been forced up from below, since it is
sharply differentiated from the surrounding country rocks. This blue
ground is a brecciated peridotite of peculiar constitution, to which
the well-known petrologist, Carvil Lewis, who made a careful study
of it, gave the name kimberlite. The blue colour testifies to its
richness in iron, and it is to the oxidation of the iron constituent,
that the change of colour to yellow in the upper levels is due. Owing
to the shafts that have been sunk for working the mines, the nature
of the surrounding rocks is known to some depth. Immediately below
the surface is a decomposed ferriferous basalt, about 20 to 90 feet
(6-27 m.) thick, next a black slaty shale, 200 to 250 feet (60-75 m.)
thick, then 10 feet (3 m.) of conglomerate, next 400 feet (120 m.)
of olivine diabase, then quartzite, about 400 feet (120 m.) thick,
and lastly a quartz porphyry, which has not yet been penetrated. The
strata run nearly horizontal, and there are no signs of upward bending
at the pipes. The whole of the country, including the mines, was
covered with a red sandy soil, and there was nothing to indicate the
wealth that lay underneath. The action of water had in process of time
removed all signs of eruptive activity. The principal minerals which
are associated with diamond in the blue ground are magnetite, ilmenite,
chromic pyrope, which is put on the market as a gem under the misnomer
‘Cape-ruby,’ ferriferous enstatite, which also is sometimes cut,
olivine more or less decomposed, zircon, kyanite, and mica.

The evidence produced by an examination of the blue ground and the
walls of the pipes proves that the pipes cannot have been volcanoes
such as Vesuvius. There is no indication whatever of the action of any
excessive temperature, while, on the other hand, there is every sign of
the operation of enormous pressure; the diamonds often contain liquid
drops of carbonic acid. Crookes puts forward the plausible theory that
steam has been the primary agency in propelling the diamond and its
associates up into the channel through which it has carved its way to
freedom, and holds that molten iron has been the solvent for carbon
which has crystallized out as diamond under the enormous pressures
obtaining in remote depths of the earth’s crust. It is pertinent to
note that, by dissolving carbon in molten iron, the eminent chemist,
Moissan, was enabled to manufacture tiny diamond crystals. Water
trickling down from above would be immediately converted into steam at
very high pressure on coming into contact with the molten iron, and, in
its efforts to escape, the steam would drive the iron and its precious
contents, together with the adjacent rocks, upwards to the surface. The
ferriferous nature of the blue ground and the yellow tinge so common
to the diamonds lend confirmation to this theory. The process by which
the carbon was extracted from shales or other carboniferous rocks and
dissolved in iron still awaits elucidation.

Diamonds were found in New South Wales as long ago as 1851 on Turon
River and at Reedy Creek, near Bathurst, about ninety miles (145 km.)
from Sydney, but the find was of little commercial importance. A more
extensive deposit came to light in 1867 farther north at Mudgee. In
1872 diamonds were discovered in the extreme north of the State, at
Bingara near the Queensland border. Another discovery was made in 1884
at Tingha, and still more recently in the tin gravels of Inverell in
the same region. In their freedom from colour and absence of twinning
the New South Wales diamonds resemble the Brazilian stones. The average
size is small, running about five to the carat when cut; the largest
found weighed nearly 6 carats when cut. They are remarkable for their
excessive hardness; they can be cut only with their own dust, ordinary
diamond dust making no impression.

The Borneo diamonds are likewise distinguished by their exceptional
hardness. They mostly occur by the river Landak, near Pontianak on the
west coast of the island. They are found in a layer of rather coarse
gravel, variable, but rarely exceeding a yard (1 m.), in depth, and are
associated with corundum and rutile, together with the precious metals
gold and platinum. Indeed, it is no uncommon sight to see natives
wearing waistcoats ornamented with gold buttons, in each of which a
diamond is set. The diamonds are well crystallized and generally of
pure water; yellowish and canary-yellow stones are also common, but
rose-red, bluish, smoky, and black stones are rare. They seldom exceed
a carat in weight; but stones of 10 carats in weight are found, and
occasionally they attain to 20 carats. In 1850 a diamond weighing 77
carats was discovered. The Rajah of Mattan is said to possess one of
the purest water weighing as much as 367 carats, but no one qualified
to pronounce an opinion regarding its genuineness has ever seen it.

In Rhodesia small diamonds have been found in gravel beds resting on
decomposed granite near the Somabula forest, about 12 miles (19 km.)
west of Gwelo, in association with chrysoberyl in abundance, blue
topaz, kyanite, ruby, sapphire, tourmaline, and garnet.

The occurrence of diamond in German South-West Africa is very peculiar.
Large numbers of small stones are found close to the shore near
Luderitz Bay in a gravelly surface layer, which is nowhere more than a
foot in depth. They are picked by hand by natives and washed in sieves.
In shape they are generally six-faced octahedra or twinned octahedra,
simple octahedra being rare, and in size they run about four or five
to the carat, the largest stone as yet found being only 2 carats in
weight. Their colour is usually yellowish.

Several isolated finds of diamonds have been reported in California
and other parts of the United States, but none have proved of any
importance. The largest stone found weighed 23¾ carats uncut; it was
discovered at Manchester in Virginia.

CHAPTER XVIII

HISTORICAL DIAMONDS

The number of diamonds which exceed a hundred carats in weight when
cut is very limited. Their extreme costliness renders them something
more than mere ornaments; in a condensed and portable form they
represent great wealth and all the potentiality for good or ill thereby
entailed, and have played no small, if sinister, rôle in the moulding
of history. In bygone days when despotic government was universal, the
possession of a splendid jewel in weak hands but too often precipitated
the aggression of a greedy and powerful neighbour, and plunged whole
countries into the horrors of a ruthless and bloody war. In more
civilized days a great diamond has often been pledged as security for
money to replenish an empty treasury in times of stress. The ambitions
of Napoleon might have received a set-back but for the funds raised
on the security of the famous Pitt diamond. The history of such
stones—often one long romance—is full of interest, but space will not
permit of more than a brief sketch here.

If we except the colossal Cullinan stone, the mines of Brazil and South
Africa cannot compare with the old mines of India as the birthplace of
large and perfect diamonds of world-wide fame.

(1) KOH-I-NOR

[Illustration: FIG. 61.—Koh-i-nor (top view).]

[Illustration: FIG. 62.—Koh-i-nor (side view).]

The history of the famous stone called the Koh-i-nor, meaning Mound of
Light, is known as far back as the year 1304, when it fell into the
hands of the Mogul emperors, and legend even traces it back some four
thousand years previously. It remained at Delhi until the invasion of
North-West India by Nadir Shah in 1739, when it passed together with an
immense amount of spoil into the hands of the conqueror. At his death
the empire which he had so strenuously founded fell to pieces, and
the great diamond after many vicissitudes came into the possession of
Runjit Singh at Lahore. His successors kept it until upon the fall of
the Sikh power in 1850 it passed to the East India Company, in whose
name it was presented by Lord Dalhousie to Queen Victoria. At this
date the stone still retained its original Indian form, but in 1862 it
was re-cut into the form of a shallow brilliant (Fig. 62), the weight
thereby being reduced from 186-1/16 to 106-1/16 carats. The wisdom
of this course has been severely criticized; the stone has not the
correct shape of a brilliant and is deficient in ‘fire,’ and it has
with the change in shape lost much of its old historical interest.
The Koh-i-nor is the private property of the English Royal Family, the
stone shown in the Tower being a model. It is valued at £100,000.

(2) PITT OR REGENT

[Illustration: FIG. 63.—Pitt or Regent (top view).]

[Illustration: FIG. 64.—Pitt or Regent (side view).]

This splendid stone was discovered in 1701 at the famous diamond mines
at Partial, on the Kistna, about 150 miles (240 km.) from Golconda,
and weighed as much as 410 carats in the rough. By devious ways it
came into the hands of Jamchund, a Parsee merchant, from whom it was
purchased by William Pitt, governor of Fort St. George, Madras, for
£20,400. On his return to England Pitt had it cut into a perfect
brilliant (Fig. 63), weighing 163⅞ carats, the operation occupying
the space of two years and costing £5000; more than £7000 is said to
have been realized from the sale of the fragments left over. Pitt
had an uneasy time and lived in constant dread of theft of the stone
until, in 1717, after lengthy negotiations, he parted with it to the
Duc d’Orléans, Regent of France, for the immense sum of three and
three-quarter million francs, about £135,000. With the remainder of the
French regalia it was stolen from the Garde-meuble on August 17, 1792,
in the early days of the French Revolution, but was eventually restored
by the thieves, doubtless because of the impossibility of disposing of
such a stone, at least intact, and it is now exhibited in the Apollo
Gallery of the Louvre at Paris. It measures about 30 millimetres in
length, 25 in width, and 19 in depth, and is valued at £480,000.

(3) ORLOFF

[Illustration: FIG. 65.—Orloff (top view).]

[Illustration: FIG. 66.—Orloff (side view).]

One of the finest diamonds existing, this large stone forms the top
of the imperial sceptre of Russia. It is rose-cut (Fig. 65), the base
being a cleavage face, and weighs 194¾ carats. It is said to have
formed at one time one of the eyes of a statue of Brahma which stood
in a temple on the island of Sheringham in the Cavery River, near
Trichinopoli, in Mysore, and to have been stolen by a French soldier
who had somehow persuaded the priests to appoint him guardian of the
temple. He sold it for £2000 to the captain of an English ship, who
disposed of it to a Jewish dealer in London for £12,000. It changed
hands to a Persian merchant, Raphael Khojeh, who eventually sold it to
Prince Orloff for, so it is said, the immense sum of £90,000 and an
annuity of £4000. It was presented by Prince Orloff to Catherine II of
Russia.

(4) GREAT MOGUL

This, the largest Indian diamond known, was found in the Kollur mines,
about the year 1650. Its original weight is said to have been 787½
carats, but it was so full of flaws that the Venetian, Hortensio
Borgis, then in India, in cutting it to a rose form reduced its weight
to 240 carats. It was seen by Tavernier at the time of his visit
to India, but it has since been quite lost sight of. It has been
identified with both the Koh-i-nor and the Orloff, and it is even
suggested that both these stones were cut from it.

(5) SANCY

The history of this diamond is very involved, and probably two or more
stones have been confused. It may have been the one cut by Berquem for
Charles the Bold, from whose body on the fatal day of Nancy, in 1477,
it was snatched by a marauding soldier. It was acquired by Nicholas
Harlai, Seigneur de Sancy, who sold it to Queen Elizabeth at the close
of the sixteenth century. A hundred years later, in 1695, it was sold
by James II to Louis XIV. The stone in the French regalia, according
to the inventory taken in 1791, weighed 53¾ carats. It was never
recovered after the theft of the regalia in the following year, but
may be identical with the diamond which was in the possession of the
Demidoff family and was sold by Prince Demidoff in 1865 to a London
firm who were said to have been acting for Sir Jamsetjee Jeejeebhoy, a
wealthy Parsee of Bombay. It was shown at the Paris Exhibition of 1867.
It was almond-shaped, and covered all over with tiny facets by Indian
lapidaries.

(6) GREAT TABLE

This mysterious stone was seen by Tavernier at Golconda in 1642, but
has quite disappeared. It weighed 242-3/16 carats.

(7) MOON OF THE MOUNTAINS

This diamond is often confused with the Orloff. It was captured by
Nadir Shah at Delhi, and after his murder was stolen by an Afghan
soldier who disposed of it to an Armenian, by name Shaffrass. It was
finally acquired by the Russian crown for an enormous sum.

(8) NIZAM

A large diamond, weighing 340 carats, belonged to the Nizam of
Hyderabad; it was fractured at the beginning of the Indian Mutiny.
Whether the weight is that previous to fracture or not, there seems to
be no information.

(9) DARYA-I-NOR

This fine diamond, rose-cut and 186 carats in weight, is of the purest
water and merits its title of ‘River of Light.’ It seems to have been
captured by Nadir Shah at Delhi, and is now the largest diamond in the
Persian collection.

(10) SHAH

This fine stone, of the purest water, was presented to the Czar
Nicholas by the Persian prince Chosroes, younger son of Abbas Mirza, in
1843. At that time it still retained three cleavage faces which were
engraved with the names of three Persian sovereigns, and weighed 95
carats. It was, however, subsequently re-cut with the loss of 9 carats,
and the engraving has disappeared in the process.

(11) AKBAR SHAH, OR JEHAN GHIR SHAH

Once the property of the great Mogul, Akbar, this diamond was engraved
on two faces with Arabic inscriptions by the instructions of his
successor, Jehan. It disappeared, but turned up again in Turkey
under the name of ‘Shepherd’s Stone’; it still retained its original
inscriptions and was thereby recognized. In 1866 it was re-cut, the
weight being reduced from 116 to 71 carats, and the inscriptions
destroyed. The stone was sold to the Gaekwar of Baroda for 3½ lakhs of
rupees (about £23,333).

(12) POLAR STAR

A beautiful, brilliant-cut stone, weighing 40 carats, which is known by
this name, is in the Russian regalia.

(13) NASSAK

The Nassak diamond, which weighed 89¾ carats, formed part of the
Deccan booty, and was put up to auction in London in July 1837. It
was purchased by Emanuel, a London jeweller, who for £7200 shortly
afterwards sold it to the Duke of Westminster, in whose family it still
remains. It was originally pear-shaped, but was re-cut to a triangular
form with a reduction in weight to 78⅝ carats.

(14) NAPOLEON

This diamond was purchased by Napoleon Buonaparte for £8000, and worn
by him at his wedding with Josephine Beauharnais in 1796.

(15) CUMBERLAND

This stone, which weighs 32 carats, was purchased by the city of London
for £10,000 and presented to the Duke of Cumberland after the battle of
Culloden; it is now in the possession of the Duke of Brunswick.

(16) PIGOTT

A fine Indian stone, weighing 47½ carats, this diamond was brought
to England by Lord Pigott in 1775 and sold for £30,000. It came into
the possession of Ali Pacha, Viceroy of Egypt, and was by his orders
destroyed at his death.

(17) EUGÉNIE

This fine stone, weighing 51 carats, was given by the Czarina Catherine
II of Russia to her favourite, Potemkin. It was purchased by Napoleon
III as a bridal gift for his bride, and on his downfall was bought by
the Gaekwar of Baroda.

(18) WHITE SAXON

Square in contour, measuring 1-1/12 in. (28 mm.), and weighing 48¾
carats, this stone was purchased by Augustus the Strong for a million
thalers (about £150,000).

(19) PACHA OF EGYPT

This 40-carat brilliant was purchased by Ibrahim, Viceroy of Egypt, for
£28,000.

(20) STAR OF ESTE

Though a comparatively small stone, in weight 25½ carats, it is noted
for its perfection of form and quality. It belongs to the Archduke
Franz Ferdinand of Austrian-Este, eldest son of the Archduke Karl
Ludwig.

(21) TUSCANY, OR AUSTRIAN YELLOW

The beauty of this large stone, 133¾ carats in weight, is marred by
the tinge of yellow, which is sufficiently pronounced to impair its
brilliancy; it is a double rose in form. At one time the property of
the Grand Dukes of Tuscany, it is now in the possession of the Emperor
of Austria. King mentions a tale that it was bought at a curiosity
stall in Florence for an insignificant sum, the stone being supposed to
be only yellow quartz.

(22) STAR OF THE SOUTH

This, the largest of the Brazilian diamonds, was discovered at the
mines of Bagagem in July 1853. Perfectly transparent and without tint,
it was dodecahedral in shape and weighed 254½ carats, and was sold in
the rough for £40,000. It was cut as a perfect brilliant, being reduced
in weight to 125½ carats.

(23) ENGLISH DRESDEN

This beautiful stone, which weighed 119½ carats in the rough, was found
at the Bagagem mines, in Brazil, in 1857, and came into the possession
of Mr. E. Dresden. It was cut as a long, egg-shaped brilliant, weighing
76½ carats.

(24) STAR OF SOUTH AFRICA

The first considerable stone to be found in South Africa, it was
discovered at the Vaal River diggings in 1869, and weighed 83½ carats
in the rough. It was cut to a triangular brilliant of 46½ carats. It
was finally purchased by the Countess of Dudley for £25,000.

(25) STEWART

This large diamond, weighing in the rough 288⅜ carats, was found at the
Vaal River diggings in 1872, and was first sold for £6000 and shortly
afterwards for £9000; it was reduced on cutting to 120 carats. Like
many South African stones, it has a faint yellowish tinge.

(26) PORTER-RHODES

This blue-white stone, which weighed 150 carats, was found in a claim
belonging to Mr. Porter-Rhodes in the Kimberley mine in February 1880.

(27) IMPERIAL, VICTORIA, OR GREAT WHITE

This large diamond weighed as much as 457 carats in the rough, and 180
when cut; it is quite colourless. It was brought to Europe in 1884, and
was eventually sold to the Nizam of Hyderabad for £20,000.

(28) DE BEERS

A pale yellowish stone, weighing 428½ carats, was found in the De Beers
mine in 1888. It was cut to a brilliant weighing 228½ carats, and
was sold to an Indian prince. A still larger stone of similar tinge,
weighing 503¼ carats, was discovered in 1896, and among other large
stones supplied by the same mine may be mentioned one of 302 carats
found in 1884, and another of 409 carats found in early years.

(29) EXCELSIOR

This, which prior to the discovery of the ‘Cullinan,’ was by far the
largest South African stone, was found in the Jagersfontein mine on
June 30, 1893; bluish-white in tint, it weighed 969½ carats. From
it were cut twenty-one brilliants, the larger stones weighing 67⅞,
45-13/16, 45-11/16, 39-3/16, 34, 27⅞, 25⅝, 23-11/16, 16-11/32, 13½
carats respectively, and the total weight of the cut stones amounting
to 364-3/32 carats.

(30) JUBILEE

Another large stone was discovered in the Jagersfontein mine in
1895. It weighed 634 carats in the rough, and from it was obtained a
splendid, faultless brilliant weighing 239 carats. It was shown at the
Paris Exhibition of 1900.

(31) STAR OF AFRICA, OR CULLINAN

[Illustration: FIG. 67.—Cullinan No. 1.]

All diamonds pale into insignificance when compared with the colossal
stone that came to light at the Premier mine near Pretoria in the
Transvaal on January 25, 1905. It was first called the ‘Cullinan’ after
Sir T. M. Cullinan, chairman of the Premier Diamond Mine (Transvaal)
Company, but has recently, by desire of King George V, received the
name ‘Star of Africa.’ The rough stone weighed 621·2 grams or 3025¾
carats (about 1⅓ lb.); it displayed three natural faces (Plate XXV)
and one large cleavage face, and its shape suggested that it was
a portion of an enormous stone more than double its size; it was
transparent, colourless, and had only one small flaw near the surface.
This magnificent diamond was purchased by the Transvaal Government for
£150,000, and presented to King Edward VII on his birthday, November 9,
1907.

[Illustration: _PLATE XXV_

CULLINAN DIAMOND

(_Natural size_)]

[Illustration: FIG. 68.—Cullinan No. 2.]

The Cullinan was entrusted to the famous firm, Messrs. I. J. Asscher
& Co., of Amsterdam, for cutting on January 23, 1908, just three
years after its discovery. On February 10 it was cleaved into two
parts, weighing respectively 1977½ and 1040½ carats, from which the
two largest stones have been cut, one being a pendeloque or drop
brilliant in shape (Fig. 67) and weighing 516½ carats, and the other
a square brilliant (Fig. 68) weighing 309-3/16 carats. The first
has been placed in the sceptre, and the second in the crown of the
regalia. Besides these there are a pendeloque weighing 92 carats, a
square-shaped brilliant 62, a heart-shaped stone 18⅜, two marquises
8-9/16 and 11¼, an oblong stone 6⅝, a pendeloque 4-9/32, and 96 small
brilliants weighing together 7⅜; the total weight of the cut stones
amounts to 1036-5/32 carats. The largest stone has 74 and the second 66
facets. The work was completed and the stones handed to King Edward in
November 1908.

Although the Premier mine has yielded no worthy compeer of the
Cullinan, it can, nevertheless, boast of a considerable number of
large stones which but for comparison with that giant would be thought
remarkable for their size, no fewer than seven of them having weights
of over 300 carats, viz. 511, 487¼, 458¾, 391½, 373, 348, and 334
carats.

(32) STAR OF MINAS

This large diamond, which was found in 1911 at the Bagagem mines, Minas
Geraes, Brazil, had the shape of a dome with a flat base, and weighed
in the rough 35·875 grams (174¾ carats).

• • • • •

The large stone called the ‘Braganza,’ in the Portuguese regalia, which
is supposed to be a diamond, is probably a white topaz; it weighs 1680
carats. The Mattan stone, pear-shaped and weighing 367 carats, which
was found in the Landak mines near the west coast of Borneo in 1787, is
suspected to be quartz.

COLOURED DIAMONDS

(1) HOPE

[Illustration: FIG. 69.—Hope.]

The largest of coloured diamonds, the Hope, weighs 44⅛ carats, and has
a steely- or greenish-blue, and not the royal-blue colour of the glass
models supposed to represent it. It is believed to be a portion of a
drop-form stone (_d’un beau violet_) which was said to have been found
at the Kollur mines, and was secured by Tavernier in India in 1642 and
sold by him to Louis XIV in 1668; it then weighed 67 carats. This stone
was stolen with the remainder of the French regalia in 1792 and never
recovered. In 1830 the present stone (Fig. 69) was offered for sale
by Eliason, a London dealer, and was purchased for £18,000 by Thomas
Philip Hope, a wealthy banker and a keen collector of gems. Probably
the apex of the original stone had been cut off, reducing it to a
nearly square stone. The slight want of symmetry of the present stone
lends confirmation to this view, and two other blue stones are known,
which, together with the Hope, make up the weight of the original
stone. At the sale of the Hope collection at Christie’s in 1867 the
blue diamond went to America. In 1908 the owner disposed of it to Habib
Bey for the enormous sum of £80,000. It was put up to auction in Paris
in 1909, and bought by Rosenau, the Paris diamond merchant, for the
comparatively small sum of 400,000 francs (about £16,000), and was sold
in January 1911 to Mr. Edward M’Lean for £60,000. The stone is supposed
to bring ill-luck in its train, and its history has been liberally
embellished with fable to establish the saying.

(2) DRESDEN

A beautiful apple-green diamond, faultless, and of the purest water, is
contained in the famous Green Vaults of Dresden. It weighs 40 carats,
and was purchased by Augustus the Strong in 1743 for 60,000 thalers
(about £9000).

(3) PAUL I

A fine ruby-red diamond, weighing 10 carats, is included among the
Russian crown jewels.

(4) TIFFANY

The lovely orange brilliant, weighing 125⅜ carats, which is in the
possession of Messrs. Tiffany & Co., the well-known jewellers of New
York, was discovered in the Kimberley mine in 1878.

CHAPTER XIX

CORUNDUM

(_Sapphire_, _Ruby_)

Ranking in hardness second to diamond alone, the species known to
science as corundum and widely familiar by the names of its varieties,
sapphire and ruby, holds a pre-eminent position among coloured
gem-stones. The barbaric splendour of ruby (Plate I, Fig. 13) and the
glorious hue of sapphire (Plate I, Fig. 11) are unsurpassed, and it
is remarkable that the same species should boast such different, but
equally magnificent, tints. They, however, by no means exhaust the
resources of this variegated species. Fine yellow stones (Plate I,
Fig. 12), which compare with topaz in colour and are its superior in
hardness, and brilliant colourless stones, which are unfortunately
deficient in ‘fire’ and cannot therefore approach diamond, are to
be met with, besides others of less attractive hues, purple, and
yellowish, bluish, and other shades of green. Want of homogeneity in
the coloration of corundum is a frequent phenomenon; thus, the purple
stones on close examination are found to be composed of alternate blue
and red layers, and stones showing patches of yellow and blue colour
are common. Owing to the peculiarity of their interior arrangement
certain stones display when cut _en cabochon_ a vivid six-rayed star of
light (Plate I, Fig. 15). Sapphire and ruby share with diamond, pearl,
and emerald the first rank in jewellery. They are popular stones,
especially in rings; their comparative rarity in large sizes, apart
from the question of expense, prevents their use in the bigger articles
of jewellery. The front of the stones is usually brilliant-cut and the
back step-cut, but Indian lapidaries often prefer to cover the stone
with a large number of triangular facets, especially if the stone be
flawed; star-stones are cut more or less steeply _en cabochon_.

In composition corundum is alumina, oxide of aluminium, corresponding
to the formula Al_{2}O_{3}, but it usually contains in addition small
quantities, rarely more than 1 per cent., of ferric oxide, chromic
oxide, and perhaps other metallic oxides. When pure, it is colourless;
the splendid tints which are its glory have their origin in the minute
traces of the other oxides present. No doubt chromic oxide is the
cause of the ruddy hue of ruby, since it is possible, as explained
above (p. 117), closely to imitate the ruby tint by this means, but
nothing approaching so large a percentage as 2½ has been detected in
a natural stone. The blue colour of sapphire may be due to titanic
oxide, and ferric oxide may be responsible for the yellow hue of
the ‘oriental topaz,’ as the yellow corundum is termed. Sapphires,
when of considerable size, are rarely uniform in tint throughout the
stone. Alternations of blue and red zones, giving rise to an apparent
purple or violet tint, and the conjunction of patches of blue and
yellow are common. Perfectly colourless stones are less common, a
slight bluish tinge being usually noticeable, but they are not in much
demand because, on account of their lack of ‘fire,’ they are of little
interest when cut. The tint of the red stones varies considerably in
depth; jewellers term them, when pale, pink sapphires, but, of course,
no sharp distinction can be drawn between them and rubies. The most
highly prized tint is the so-called pigeon’s blood, a shade of red
slightly inclined to purple. The prices for ruby of good colour run
from about 25s. a carat for small stones to between £60 and £80 a
carat for large stones, and still higher for exceptional rubies. The
taste in sapphires has changed of recent times. Formerly the deep blue
was most in demand, but now the lighter shade, that resembling the
colour of corn-flower, is preferred, because it retains a good colour
in artificial light. Large sapphires are more plentiful than large
rubies, and prices run lower; even for large perfect stones the rate
does not exceed £30 a carat. Large and uniform ‘oriental topazes’ are
comparatively common, and realize moderate prices, about 2s. to 30s.
a carat according to quality and size. Green sapphires are abundant
from Australia, but their tint, a kind of deep sage-green, is not very
pleasing. Brown stones with a silkiness of structure are also known.

The name of the species comes through the French _corindon_ from an
old Hindu word, _korund_, of unknown significance, and arose from the
circumstance that the stones which first found their way to Europe came
from India. At the present day the word corundum is applied in commerce
to the opaque stones used for abrasive purposes, to distinguish the
purer material from emery, which is corundum mixed with magnetite and
other heavy stones of lower hardness. The origin of the word sapphire,
which means blue, has been discussed in an earlier chapter (p. 110).
Jewellers use it in a general sense for all corundum except ruby. Ruby
comes from the Latin _ruber_, red. The prefix ‘oriental’ (p. 111)
is often used to distinguish varieties of corundum, since it is the
hardest of ordinary coloured stones and the finest gem-stones in early
days reached Europe by way of the East.

Corundum crystallizes either in six-sided prisms terminated by flat
faces (Plate I, Fig. 10), which are often triangularly marked, or with
twelve inclined faces, six above and six below, meeting in a girdle
(Plate I, Fig. 14). Ruby favours the former and the other varieties
the latter type. A fine crystal of ruby—the ‘Edwardes,’ so named by
the donor, John Ruskin, after Sir Herbert Edwardes—which weighs 33·5
grams (163 carats), is exhibited in the Mineral Gallery of the British
Museum (Natural History), and is tilted in such a way that the light
from a neighbouring window falls on the large basal face, and reveals
the interesting markings that nature has engraved on it. From its type
of symmetry corundum is doubly refractive with a direction of single
refraction running parallel to the edge of the prism. Owing to the
relative purity of the chemical composition the refractive indices
are very constant; the ordinary index ranges from 1·766 to 1·774 and
the extraordinary index from 1·757 to 1·765, the double refraction
remaining always the same, 0·009. The amount of colour-dispersion is
small, and therefore colourless corundum displays very little ‘fire.’
The difference between the indices for red and blue light is, however,
sufficiently great that the base of a ruby may be left relatively
thicker than that of a sapphire to secure an equally satisfactory
effect (cf. p. 98)—a point of some importance to the lapidary, since
stones are sold by weight and it is his object to keep the weight as
great as possible. When a corundum is tested on the refractometer in
white light a wide spectrum deliminates the two portions of the field
because of the smallness of the colour-dispersion (cf p. 25). The
dichroism of both ruby and sapphire is marked, the twin colours given
by the former being red and purplish-red, and by the latter blue and
yellowish-blue, the second colour in each instance corresponding to
the extraordinary ray. Tests with the dichroscope easily separate ruby
and sapphire from any other red or blue stone. This character has an
important bearing on the proper mode of cutting the stones. The ugly
yellowish tint given by the extraordinary ray of sapphire should be
avoided by cutting the stone with its table-facet at right angles to
the prism edge, which is the direction of single refraction. Whether
a ruby should be treated in the same way is a moot point. No doubt
if the colour is deep, it is the best plan, because the amount of
absorption of light is thereby sensibly reduced, but otherwise the
delightful nuances distinguishing ruby are best secured by cutting
the table-facet parallel to the direction of single refraction.
Yellow corundum also shows distinct dichroism, but by a variation
more of the depth than of the tint of the colour; the phenomenon is
faint compared with the dichroic effect of a yellow chrysoberyl. The
specific gravity also is very constant, varying only from 3·95 to 4·10;
sapphire is on the whole lighter than ruby. Corundum has the symbol 9
on Mohs’s scale, but though coming next to diamond it is a very poor
second (cf. p. 79). As is usually the case, the application of heat
tends to lighten the colour of the stones: those of a pale violet or
a yellow colour lose the tint entirely, and the deep violet stones
turn a lovely rose colour. On the other hand the action of radium has,
as was shown by Bordas, an intensifying action on the colour, and
even develops it in a colourless stone. From the latter reaction it
may be inferred that often in an apparently colourless stone two or
more selective influences are at work which ordinarily neutralize one
another, but, being unequally stimulated by the action of radium, they
thereupon give rise to colour. The stellate appearance of asterias
or star-stones—star-ruby and star-sapphire—results from the regular
arrangement either of numerous small channels or of twin-lamellæ in the
stone parallel to the six sides of the prisms; light is reflected from
the interior in the form of a six-rayed star (p. 38). Some stones from
Siam possess a markedly fibrous or silky structure.

The synthetical manufacture of ruby, sapphire, and other varieties of
corundum has already been described (p. 116).

Besides its use in jewellery corundum is on account of its hardness of
great service for many other purposes. Small fragments are extensively
employed for the bearing parts of the movements of watches, and both
the opaque corundum and the impure kind known as emery are in general
use for grinding and polishing softer stones, and steel and other
metal-work.

The world’s supply of fine rubies is drawn almost entirely from the
famous ruby mines near Mogok, situated about 90 miles (145 km.) in
a north-easterly direction from Mandalay in Upper Burma and at an
elevation of about 4000 ft. (1200 m.) above sea-level. It is from this
district that the stones of the coveted carmine-red, the so-called
‘pigeon’s blood,’ colour are obtained. The ruby occurs in a granular
limestone or calcite in association with the spinel of nearly the
same appearance—the ‘balas-ruby,’ oriental topaz (yellow corundum),
tourmaline, and occasionally sapphire. Some stones are found in
the limestone on the sides of the hills, but by far the largest
quantity occur in the alluvial deposits, both gravel and clay, in the
river-beds; the ruby ground is locally known as ‘_byon_.’ The stones
are as a rule quite small, averaging only about four to the carat.
Before the British annexation of the country in 1885 the mines were a
monopoly of the Burmese sovereigns and were worked solely under royal
licence. They are known to be of great antiquity, but otherwise their
early history is a mystery. It is said that an astute king secured the
priceless territory in 1597 from the neighbouring Chinese Shans in
exchange for a small and unimportant town on the Irrawaddy; if that
be so, he struck an excellent bargain. The mines were allotted to
licensed miners, _twin-tsas_ (eaters of the mine) as they were called
in the language of the country, who not only paid for the privilege,
but were compelled to hand over to the king all stones above a certain
weight. As might be anticipated this injunction caused considerable
trouble, and the royal monopolists constantly suspected the miners of
evading the regulation by breaking up stones of exceptional size; from
subsequent experience, it is probable that large stones were in reality
seldom found. Since 1887 the mines have been worked by arrangement with
the Government of India by the Ruby Mines, Ltd., an English company.
Its career has been far from prosperous, but during recent years, in
consequence of the improved methods of working the mines and of the
more generous terms afterwards accorded by the Government, greater
success has been experienced; the future is, however, to some extent
clouded by the advent of the synthetical stone, which has even made its
way out to the East.

Large rubies are far from common, and such as were discovered in the
old days were jealously hoarded by the Burmese sovereigns. According to
Streeter the finest that ever came to Europe were a pair brought over
in 1875, at a time when the Burmese king was pressed for money. One,
rich in colour, was originally cushion-shaped and weighed 37 carats;
the other was a blunt drop in form and weighed 47 carats. Both were cut
in London, the former being reduced to 32-5/16 carats and the latter
to 38-9/16 carats, and were sold for £10,000 and £20,000 respectively.
A colossal stone, weighing 400 carats, is reported to have been found
in Burma; it was broken into three pieces, of which two were cut and
resulted in stones weighing 70 and 45 carats respectively, and the
third was sold uncut in Calcutta for 7 lakhs of rupees (£46,667). The
finder of another large stone broke it into two parts, which after
cutting weighed 98 and 74 carats respectively; he attempted in vain to
evade the royal acquisitiveness, by giving up the larger stone to the
king and concealing the other. A fine stone, known by the formidable
appellation of ‘Gnaga Boh’ (Dragon Lord), weighed 44 carats in the
rough and 20 carats after cutting. Since the mines were taken over
by the Ruby Mines, Ltd., a few large stones have been discovered. A
beautiful ruby was found in the Tagoungnandaing Valley, and weighed
18½ carats in the rough and 11 carats after cutting; perfectly clear
and of splendid colour, it was sold for £7000, but is now valued at
£10,000. Another, weighing 77 carats in the rough, was found in 1899,
and was sold in India in 1904 for 4 lakhs of rupees (£26,667). A stone,
weighing 49 carats, was discovered in 1887, and an enormous one,
weighing as much as 304 carats, in 1890.

The ruby, as large as a pigeon’s egg, which is amongst the Russian
regalia was presented in 1777 to the Czarina Catherine by Gustav III of
Sweden when on a visit to St. Petersburg. The large red stone in the
English regalia which was supposed to be a ruby is a spinel (cf p. 206).

Comparatively uncommon as sapphires are in the Burma mines a faultless
stone, weighing as much as 79½ carats, has been discovered there.

Good rubies, mostly darker in colour than the Burmese stones, are
found in considerable quantity near Bangkok in Siam, Chantabun
being the centre of the trade, where, just as in Burma, they are
intimately associated with the red spinel. Because of the difference
in tint and the consequent difference in price, jewellers draw a
distinction between Burma and Siam rubies; but that, of course, does
not signify any specific difference between them. Siam is, however,
most distinguished as the original home of splendid sapphires. The
district of Bo Pie Rin in Battambang produces, indeed, more than
half the world’s supply of sapphires. In the Hills of Precious
Stones, such being the meaning of the native name for the locality, a
number of green corundums are found. Siam also produces brown stones
characterized by a peculiar silkiness of structure. Rubies are found in
Afghanistan at the Amir’s mines near Kabul and also to the north of the
lapis lazuli mines in Badakshan.

The conditions in Ceylon are precisely the converse of those obtaining
in Burma; sapphire is plentiful and ruby rare in the island. They are
found in different rocks, sapphire occurring with garnet in gneiss, and
ruby accompanying spinel in limestone, but they come together in the
resulting gravels, the principal locality being the gem-district near
Ratnapura in the south of the island. The largest uncut ruby discovered
in Ceylon weighed 42½ carats; it had, however, a decided tinge of
blue in it. Ceylon is also noted for the magnificent yellow corundum,
‘oriental topaz,’ or, as it is locally called, ‘king topaz,’ which it
produces.

Beautiful sapphires occur in various parts of India, but particularly
in the Zanskar range of the north-western Himalayas in the state of
Kashmir, where they are associated with brown tourmaline. Probably most
of the large sapphires known have emanated from India. By far the most
gigantic ever reported is one, weighing 951 carats, said to have been
seen in 1827 in the treasury of the King of Ava. The collection at the
Jardin des Plantes contains two splendid rough specimens; one, known as
the ‘Rospoli,’ is quite flawless and weighs 132-1/16 carats, and the
other is 2 inches in length and 1½ inches in thickness. The Duke of
Devonshire possesses a fine cut stone, weighing 100 carats, which is
brilliant-cut above and step-cut below the girdle. An image of Buddha,
which is cut out of a single sapphire, is exhibited, mounted on a gold
pin, in the Mineral Gallery of the British Museum (Natural History).

For some years past a large quantity of sapphires have come into the
market from Montana, U.S.A., especially from the gem-district about
twelve miles west of Helena. The commonest colour is a bluish green,
generally pale, but blue, green, yellow and occasionally red stones are
also found; they are characterized by their almost metallic lustre.
With them are associated gold, colourless topaz, kyanite, and a
beautiful red garnet which is found in grains and usually mistaken for
ruby. Rubies are also found in limestone at Cowee Creek, North Carolina.

Blue and red corundum, of rather poor quality, has come from the
Sanarka River, near Troitsk, and from Miask, in the Government of
Orenburg, Russia, and similar stones have been known at Campolongo, St.
Gothard, Switzerland.

The prolific gem-district near Anakie, Queensland, supplies examples of
every known variety of corundum except ruby; blue, green, yellow, and
parti-coloured stones, and also star-stones, are plentiful. Leaf-green
corundum is known father south, in Victoria. The Australian sapphire is
too dark to be of much value.

Small rubies and sapphires are found in the gem-gravels near the
Somabula Forest, Rhodesia.

CHAPTER XX

BERYL

(_Emerald_, _Aquamarine_, _Morganite_)

The species to be considered in this chapter includes the varieties
emerald and aquamarine, as well as what jewellers understand by beryl.
It has many incontestable claims on the attention of all lovers of the
beautiful in precious stones. The peerless emerald (Plate I, Fig. 5),
which in its verdant beauty recalls the exquisite lawns that grace the
courts and quadrangles of our older seats of learning, ranks to-day
as the most costly of jewels. Its sister stone, the lovely aquamarine
(Plate I, Fig. 4), which seems to have come direct from some mermaid’s
treasure-house in the depths of a summer sea, has charms not to be
denied. Pliny, speaking of this species, truly says, “There is not a
colour more pleasing to the eye”; yet he knew only the comparatively
inferior stones from Egypt, and possibly from the Ural Mountains.
Emeralds are favourite ring-stones, and would, no doubt, be equally
coveted for larger articles of jewellery did not the excessive cost
forbid, and nothing could be more attractive for a central stone than
a choice aquamarine of deep blue-green hue. Emeralds are usually
step-cut, though Indian lapidaries often favour the _en cabochon_
form; aquamarines, on the other hand, are brilliant-cut in front and
step-cut at the back.

Beryl, to use the name by which the species is known to science, is
essentially a silicate of aluminium and beryllium corresponding to the
formula, Be_{3}Al_{2}(SiO_{3})_{6}. The beryllia is often partially
replaced by small amounts of the alkaline earths, caesia, potash, soda,
and lithia, varying from about 1½ per cent. in beryl from Mesa Grande
to nearly 5 in that from Pala and Madagascar, and over 6, of which 3·6
is caesia, in beryl from Hebron, Maine; also, as usual, chromic and
ferric oxides take the place of a little alumina; from 1 to 2 per cent.
of water has been found in emerald. The element beryllium was, as its
name suggests, first discovered in a specimen of this species, the
discovery being made in 1798 by the chemist Vauquelin; it is also known
as glucinum in allusion to the sweet taste of its salts.

When pure, beryl is colourless, but it is rarely, if ever, free
from a tinge of blue or green. The colour is usually some shade of
green—grass-green, of that characteristic tint which is in consequence
known as emerald-green, or blue-green, yellowish green (Plate I, Fig.
6), and sometimes yellow, pink, and rose-red. The peculiar colour of
emerald is supposed to be caused by chromic oxide, small quantities
of which have been detected in it by chemical analysis; moreover,
experiment shows that glass containing the same percentage amount of
chromic oxide assumes the same splendid hue. Emerald, on being heated,
loses water, but retains its colour unimpaired, which cannot therefore
be due, as has been suggested, to organic matter. The term aquamarine
is applied to the deep sea-green and blue-green stones, and jewellers
restrict the term beryl to paler shades and generally other colours,
such as yellow, golden, and pink, but Kunz has recently proposed the
name morganite to distinguish the beautiful rose beryl such as is
found in Madagascar. The varying shades of aquamarine are due to the
influence of the alkaline earths modified by the presence of ferric
oxide or chromic oxide; the beautiful blushing hue of morganite is no
doubt caused by lithia.

[Illustration: FIG. 70.—Emerald Crystal.]

The name of the species is derived from the Greek βήρυλλος, an ancient
word, the meaning of which has been lost in the mists of time. The
Greek word denoted the same species in part as that now understood
by the name. Emerald is derived from a Persian word which appeared
in Greek as σμάραγδος, and in Latin as _smaragdus_; it originally
denoted chrysocolla, or similar green stone, but was transferred upon
the introduction of the deep-green beryl from Upper Egypt. The name
aquamarine was suggested by Pliny’s exceedingly happy description of
the stones “which imitate the greenness of the clear sea,” although it
was not actually used by him. That emerald and beryl were one species
was suspected by Pliny, but the identity was not definitely established
till about a century ago. Morganite is named after John Pierpont Morgan.

The natural crystals have the form of a six-sided prism, and in the
case of emerald (Fig. 70, and Plate I, Fig. 8) invariably, if whole,
end in a single face at right angles to the length of the prism;
aquamarines have in addition a number of small inclined faces, and
stones from both Russia and Brazil often taper owing to the effects
of corrosion. The sixfold character of the crystalline symmetry
necessarily entails that the double refraction, which is small in
amount, 0·006, is uniaxial in character, and, since the ordinary
is greater than the extraordinary refractive index, it is negative
in sign. The values of the indices range between 1·567 and 1·590,
and 1·572 and 1·598 respectively, in the two cases, the pink beryl
possessing the highest values. The dichroism is distinct in the South
American emerald, the twin colours being yellowish and bluish green,
but otherwise is rather faint. The specific gravity varies between
2·69 and 2·79, and is therefore a little higher than that of quartz.
If, therefore, a beryl and a quartz be floating together in a tube
containing a suitable heavy liquid, the former will always be at a
sensibly lower level (cf. Fig. 32). The hardness varies from 7½ to
8, emerald being a little softer than the other varieties. There is
no cleavage, but like most gem-stones beryl is very brittle, and can
easily be fractured. Stones rendered cloudy by fissures are termed
‘mossy.’ When heated before the blowpipe beryl is fusible with
difficulty; it resists the attack of hydrofluoric acid as well as of
ordinary acids.

In all probability the whole of the emeralds known in ancient times
came from the so-called Cleopatra emerald mines in Upper Egypt. For
some reason they were abandoned, and their position was so completely
lost that in the Middle Ages it was maintained that emeralds had never
been found in Egypt at all, but had come from America by way of the
East. All doubts were set at rest by the re-discovery of the mines
early last century by Cailliaud, who had been sent by the Viceroy of
Egypt to search for them. They were, however, not much worked, and
after a few years were closed again, and were re-opened only about ten
years ago. The principal mines are at Jebel Zabara and at Jebel Sikait
in northern Etbai, about 10 miles (16 km.) apart and distant about 15
miles (24 km.) from the Red Sea, lying in the range of mountains that
run for a long distance parallel to the west coast of the Red Sea and
rise to over 1800 feet (550 m.) above sea-level. There are numerous
signs of considerable, but primitive, workings at distinct periods.
Both emeralds and beryls are found in micaceous and talcose schists.
The emeralds are not of very good quality, being cloudy and rather
light in colour. Finer emeralds have been found in a dark mica-schist,
together with other beryllium minerals, chrysoberyl and phenakite, and
also topaz and tourmaline on the Asiatic side of the Ural Mountains,
near the Takowaja River, which flows into the Bolshoi Reft River, one
of the larger tributaries of the Pyschma River, about fifty miles (80
km.) east of Ekaterinburg, a town which is chiefly concerned with the
mining and cutting of gem-stones. The mine was accidentally discovered
by a peasant, who noticed a few green stones at the foot of an uprooted
tree in 1830. Two years later the mine was regularly worked, and
remained open for twenty years, when it was closed. It has recently
been re-opened owing to the high rates obtaining for emeralds. Very
large crystals have been produced here, but in colour they are much
inferior to the South American stones; small Siberian emeralds, on the
other hand, are of better colour than small South American emeralds,
the latter being not so deep in tint. Emeralds have been found in a
similar kind of schist at Habachtal, in the Salzburg Alps. About thirty
years ago well-formed green stones were discovered with hiddenite at
Stony Point, Alexander County, in North Carolina, but not much gem
material has come to light.

The products of none of the mines that have just been mentioned can
on the whole compare with the beautiful stones which have come from
South America. At the time when the Spaniards grimly conquered Peru
and ruthlessly despoiled the country of the treasures which could be
carried away, immense numbers of emeralds—some of almost incredible
size—were literally poured into Spain, and eventually found their
way to other parts of Europe. These stones were known as Spanish or
Peruvian emeralds, but in all probability none of them were actually
mined in Peru. Perhaps the most extraordinary were the five choice
stones which Cortez presented to his bride, the niece of the Duke de
Bejar, thereby mortally offending the Queen, who had desired them for
herself, and which were lost in 1529 when Cortez was shipwrecked on
his disastrous voyage to assist Charles V at the siege of Algiers.
All five stones had been worked to divers fantastic shapes. One was
cut like a bell with a fine pearl for a tongue, and bore on the rim,
in Spanish, “Blessed is he who created thee.” A second was shaped
like a rose, and a third like a horn. A fourth was fashioned like a
fish, with eyes of gold. The fifth, which was the most valuable and
the most remarkable of all, was hollowed out into the form of a
cup, and had a foot of gold; its rim, which was formed of the same
precious metal, was engraved with the words, “Inter natos mulierum non
surrexit major.” As soon as the Spaniards had seized nearly all the
emeralds that the natives had amassed in their temples or for personal
adornment, they devoted their attention to searching for the source
of these marvels of nature, and eventually in 1558 they lighted by
accident upon the mines in what is now the United States of Colombia,
which have been worked almost continuously since that time. Since the
natives, who naturally resented the gross injustice with which they
had been treated, and penetrated the greed that prompted the actions
of the Spaniards, hid all traces of the mines, and refused to give any
information as to their position, it is possible that other emerald
mines may yet be found. The present mines are situated near the
village of Muzo, about 75 miles (120 km.) north-north-west of Bogota,
the capital of Colombia. The emeralds occur in calcite veins in a
bituminous limestone of Cretaceous age. The Spaniards formerly worked
the mines by driving adits through the barren rock on the hillsides to
the gem-bearing veins, but at the present day the open cut method of
working is employed. A plentiful supply of water is available, which
is accumulated in reservoirs and allowed at the proper time to sweep
the debris of barren rock away into the Rio Minero, leaving the rock
containing the emeralds exposed. Stones, of good quality, which are
suited for cutting, are locally known as _canutillos_, inferior stones,
coarse or ill-shaped, being called _morallons_.

Emerald, unlike some green stones, retains its purity of colour in
artificial light; in fact, to quote the words of Pliny, “For neither
sun nor shade, nor yet the light of candle, causeth to change and lose
their lustre.” Many are the superstitions that have been attached to
it. Thus it was supposed to be good for the eyes, and as Pliny says,
“Besides, there is not a gem or precious stone that so fully possesseth
the eye, and yet never contenteth it with satiety. Nay, if the sight
hath been wearied and dimmed by intentive poring upon anything else,
the beholding of this stone doth refresh and restore it again.” The
idea that it was fatal to the eyesight of serpents appears in Moore’s
lines—

“Blinded like serpents when they gaze
Upon the emerald’s virgin blaze.”

The crystals occur attached to the limestone, and are therefore never
found doubly terminated. The crystal form is very simple, merely a
hexagonal prism with a flat face at the one end at right angles to it.
They are invariably flawed, so much so that a flawless emerald has
passed into proverb as unattainable perfection. The largest single
crystal which is known to exist at the present day is in the possession
of the Duke of Devonshire (Fig. 71). In section it is nearly a regular
hexagon, about 2 inches (51 mm.) in diameter from side to side, and
the length is about the same; its weight is 276·79 grams (9¾ oz. Av.,
or 1347 carats). It is of good colour, but badly flawed. It was given
to the Duke of Devonshire by Dom Pedro of Brazil, and was exhibited
at the Great Exhibition of 1851. A fine, though much smaller crystal,
but of even better colour, which weighs 32·2 grams (156½ carats), and
measures 1⅛ inch (28 mm.) in its widest cross-diameter, and about the
same in length, was acquired with the Allan-Greg collection by the
British Museum, and is exhibited in the Mineral Gallery of the British
Museum (Natural History). The finest cut emerald is said to be one
weighing 30 carats, which belongs to the Czar of Russia. A small, but
perfect and flawless, faceted emerald, which is set in a gold hoop, is
also in the British Museum (Natural History). It is shown, without the
setting, about actual size, on Plate I, Fig. 5.

[Illustration: FIG. 71.—Duke of Devonshire’s Emerald.

(Natural size.)]

The ever great demand and the essentially restricted supply have forced
the cost of emeralds of good quality to a height that puts large stones
beyond the reach of all but a privileged few who have purses deep
enough. The rate per carat may be anything from £15 upwards, depending
upon the purity of the colour and the freedom from flaws, but it
increases very rapidly with the size, since flawless stones of more
than 4 carats or so in weight are among the rarest of jewels; a perfect
emerald of 4 carats may easily fetch £1600 to £2000. It seems anomalous
to say that it has never been easier to procure fine stones than during
recent years, but the reason is that the high prices prevailing have
tempted owners of old jewellery to realize their emeralds. On the other
hand, pale emeralds are worth only a nominal sum.

The other varieties of beryl are much less rare, and, since they
usually attain to more considerable, and sometimes even colossal, size,
far larger stones are obtainable. An aquamarine, particularly of good
deep blue-green colour, is a stone of great beauty, and it possesses
the merit of preserving its purity of tint in artificial light. It is a
favourite stone for pendants, brooches, and bracelets, and all purposes
for which a large blue or green stone is desired. The varying tints are
said to be due to the presence of iron in different percentages, and
possibly in different states of oxidation. Unlike emerald, the other
varieties are by no means so easily recognized by their colour. Blue
aquamarines may easily be mistaken for topaz, or vice versa, and the
yellow beryl closely resembles other yellow stones, such as quartz,
topaz, or tourmaline. Stones which are colourless or only slightly
tinted command little more than the price of cutting, but the price of
blue-green stones rapidly advances with increasing depth of tint up
to £2 a carat. The enormous cut aquamarine which is exhibited in the
Mineral Gallery of the British Museum (Natural History), affords some
idea of the great size such stones reach; a beautiful sea-green in
colour, it weighs 179·5 grams (875 carats), and is table-cut with an
oval contour.

The splendid six-sided columns which have been discovered in various
parts of Siberia are among the most striking specimens in any large
mineral collection. The neighbourhood of Ekaterinburg in the Urals
is prolific in varieties of aquamarine; especially at Mursinka have
fine stones been found, in association with topaz, amethyst, and
schorl, the black tourmaline. Good stones also occur in conjunction
with topaz at Miask in the Government of Orenburg. It is found in the
gold-washings of the Sanarka River, in the Southern Urals, but the
stones are not fitted for service as gems. Magnificent blue-green and
yellow aquamarines are associated with topaz and smoky quartz in the
granite of the Adun-Tschilon Mountains, near Nertschinsk, Transbaikal.
Stones have also been found at the Urulga River in Siberia. Most
of the bluish-green aquamarines which come into the market at the
present time have originated in Brazil, particularly in Minas Novas,
Minas Geraes, where clear, transparent stones, of pleasing colour, in
various shades, are found in the utmost profusion; beautiful yellow
stones also occur at the Bahia mines. Aquamarine was obtained in very
early times in Coimbatore District, Madras, India, and yellow beryl
comes from Ceylon. Fine blue crystals occur in the granite of the
Mourne Mountains, Ireland, but they are not clear enough for cutting
purposes; similar stones are found also at Limoges, Haute Vienne,
France. Aquamarines of various hues abound in several places in the
United States, among the principal localities being Stoneham in Maine,
Haddam in Connecticut, and Pala and Mesa Grande in San Diego County,
California. The last-named state is remarkable for the numerous stones
of varying depth of salmon-pink that have been found there. It is,
however, surpassed by Madagascar, which has recently produced splendid
stones of perfect rose-red tint and of the finest gem quality, some of
them being nearly 100 carats in weight. These stones, which have been
assigned a special name, morganite (cf. _supra_), are associated with
tourmaline and kunzite. Pink and yellow beryls and deep blue-green
aquamarines occur in the island in quantity. The pink beryls from
California are generally pale or have a pronounced salmon tint, and
seldom approach the real rose-red colour of morganite; one magnificent
rose-red crystal, weighing nearly 9 lb. (4·05 kg.), has, however, been
recently discovered in San Diego County, California, and is now in the
British Museum (Natural History). Blue-green beryl, varying in tint
from almost colourless to an emerald-green, occurs with tin-stone and
topaz about 9 miles (14½ km.) north-east of Emmaville in New South
Wales, Australia.

Probably the largest and finest aquamarine crystal ever seen was one
found by a miner on March 28, 1910, at a depth of 15 ft. (5 m.) in a
pegmatite vein at Marambaya, near Arassuahy, on the Jequitinhonha
River, Minas Geraes, Brazil. It was greenish blue in colour, and a
slightly irregular hexagonal prism, with a flat face at each end, in
form; it measured 19 in. (48·5 cm.) in length and 16 in. (41 cm.) in
diameter, and weighed 243 lb. (110·5 kg.); and its transparency was so
perfect that it could be seen through from end to end (Plate XXVI). The
crystal was transported to Bahia, and sold for $25,000 (£5133).

[Illustration: _PLATE XXVI_

LARGE AQUAMARINE CRYSTAL (_one-sixth natural size_), FOUND AT
MARAMBAYA, MINAS GERAES, BRAZIL]

PART II—SECTION B

SEMI-PRECIOUS STONES

CHAPTER XXI

TOPAZ

Topaz is the most popular yellow stone in jewellery, and often
forms the principal stone in brooches or pendants, especially in
old-fashioned articles. It is a general idea that all yellow stones
are topazes, and all topazes are yellow; but neither statement is
correct. A very large number of yellow stones that masquerade as
topaz are really the yellow quartz known as citrine. The latter is,
indeed, almost universally called by jewellers topaz, the qualification
‘Brazilian’ being used by them to distinguish the true topaz. Many
species besides those mentioned yield yellow stones. Thus corundum
includes the beautiful ‘oriental topaz’ or yellow sapphire, and yellow
tourmalines are occasionally met with; the yellow chrysoberyl always
has a greenish tinge. Topaz is generally brilliant-cut in front and
step-cut at the back, and the table facet is sometimes rounded, but
the colourless stones are often cut as small brilliants; it takes an
excellent and dazzling polish.

Topaz is a silicate of aluminium corresponding to the formula
[Al(F,OH)]_{2}SiO_{4}, which was established in 1894 by Penfield and
Minor as the result of careful research. Contrary to the general
idea, topaz is usually colourless or very pale in tint. Yellow hues
of different degrees, from pale to a rich sherry tint (Plate I, Fig.
9), are common, and pure pale blue (Plate I, Fig. 7) and pale green
stones, which often pass as aquamarine, are far from rare. Natural,
red and pink, stones are very seldom to be met with. It is, however, a
peculiarity of the brownish-yellow stones from Brazil that the colour
is altered by heating to a lovely rose-pink. Curiously, the tint is not
apparent when the stone is hot, but develops as it cools to a normal
temperature; the colour seems to be permanent. Such stones are common
in modern jewellery. Although the change in colour is accompanied by
some slight rearrangement of the constituent molecules, since such
stones are invariably characterized by high refraction and pronounced
dichroism, the crystalline symmetry, however, remaining unaltered,
the cause must be attributed to some change in the tinctorial agent,
probably oxidation. The yellow stones from Ceylon, if treated in a
similar manner, lose their colour entirely. The pale yellow-brown
stones from Russia fade on prolonged exposure to strong sunlight, for
which reason the superb suite of crystals from the Urulga River, which
came with the Koksharov collection to the British Museum, are kept
under cover.

The name of the species is derived from _topazion_ (τοπάζειν, to
seek), the name given to an island in the Red Sea, which in olden
times was with difficulty located, but it was applied by Pliny and
his contemporaries to the yellowish peridot found there. The term
was applied in the Middle Ages loosely to any yellow stone, and was
gradually applied more particularly to the stone that was then more
prevalent, the topaz of modern science. As has already been pointed out
(p. 111), the term is still employed in jewellery to signify any yellow
stone. The true topaz was probably included by Pliny under the name
_chrysolithus_.

[Illustration: FIG. 72.—Topaz Crystal.]

The symmetry is orthorhombic, and the crystals are prismatic in shape
and terminated by numerous inclined faces, and usually by a large
face perpendicular to the prism edge (Fig. 72). Topaz cleaves with
great readiness at right angles to the prism edge; owing to its facile
cleavage, flaws are easily started, and caution must be exercised
not to damage a stone by knocking it against hard and unyielding
substances. The dichroism of a yellow topaz is always perceptible,
one of the twin colours being distinctly more reddish than the other,
and the phenomenon is very marked in the case of stones the colour of
which has been artificially altered to pink. The values of the least
and the greatest of the principal indices of refraction vary from 1·615
to 1·629, and from 1·625 to 1·637, respectively, the double refraction
being about 0·010 in amount, and positive in sign. The high values
correspond to the altered stones. The specific gravity, the mean value
of which is 3·55 with a variation of 0·05 on either side, is higher
than would be expected from the refractivity. A cleavage flake exhibits
in convergent polarized light a wide-angled biaxial picture, the ‘eyes’
lying outside the field of view. The relation of the principal optical
directions and the directions of single refraction to the crystal
are shown in Fig. 27. The hardness is 8 on Mohs’s scale, and in this
character it is surpassed only by chrysoberyl, corundum, and diamond.
Topaz is pyro-electric, in which respect tourmaline alone exceeds it,
and it may be strongly electrified by friction.

Although the range of refraction overlaps that of tourmaline, there
is no risk of confusion, because the latter has nearly thrice the
amount of double refraction (cf. p. 29). Apart from the difference in
refraction, a yellow topaz ought never to be confused with a yellow
quartz, because the former sinks, and the latter floats in methylene
iodide. The same test distinguishes topaz from beryl, and, indeed, from
tourmaline also.

Judged by the criterion of price, topaz is not in the first rank of
precious stones. Stones of good colour and free from flaws are now,
however, scarce. Pale stones are worth very little, possibly less than
4s. a carat, but the price rapidly advances with increase in colour,
reaching 20s. for yellow, 80s. for pink and blue stones. Since topazes
are procurable in all sizes customary in jewellery, the rates vary but
slightly, if at all, with the size.

Topaz occurs principally in pegmatite dykes and in cavities in granite,
and is interesting to petrologists as a conspicuous instance of the
result of the action of hot acid vapours upon rocks rich in aluminium
silicates. Magnificent crystals have come from the extensive mining
district which stretches along the eastern flank of the Ural Mountains,
and from the important mining region surrounding Nertschinsk, in the
Government of Transbaikal, Siberia. Fine green and blue stones have
been found at Alabashka, near Ekaterinburg, in the Government of Perm,
and at Miask in the Ilmen Mountains, in the Government of Orenburg.
Topazes of the rare reddish hue have been picked out from the gold
washings of the Sanarka River, Troitsk, also in the Government of
Orenburg. Splendid pale-brown stones have issued from the Urulga River,
near Nertschinsk, and good crystals have come from the Adun-Tschilon
Mountains. Kamchatka has produced yellow, blue, and green stones. In
the British Isles, beautiful sky-blue, waterworn crystals have been
found at Cairngorm, Banffshire, in Scotland, and colourless stones in
the Mourne Mountains, Ireland, and at St. Michael’s Mount, Cornwall.
Most of the topazes used in jewellery of the present day come from
either Brazil or Ceylon. Ouro Preto, Villa Rica, and Minas Novas, in
the State of Minas Geraes, are the principal localities in Brazil.
Numerous stones, often waterworn, brilliant and colourless or tinted
lovely shades of blue and wine-yellow, occur there; reddish stones also
have been found at Ouro Preto. Ceylon furnishes a profusion of yellow,
light-green, and colourless, waterworn pebbles. The colourless stones
found there are incorrectly termed by the natives ‘water-sapphire,’ and
the light-green stones are sold with beryl as aquamarines; the stones
locally known as ‘king topaz’ are really yellow corundum (cf. p. 181).
Colourless crystals, sometimes with a faint tinge of colour, have
been discovered in many parts of the world, such as Ramona, San Diego
County, California, and Pike’s Peak, Colorado, in the United States,
San Luis Potosi in Mexico, and Omi and Otami-yama in Japan.

CHAPTER XXII

SPINEL

(_Balas-Ruby_, _Rubicelle_)

Spinel labours under the serious disadvantage of being overshadowed at
almost all points by its opulent and more famous cousins, sapphire and
ruby, and is not so well known as it deserves to be. The only variety
which is valued as a gem is the rose-tinted stone called balas-ruby
(Plate XXVII, Fig. 3), which is very similar to the true ruby in
appearance; they are probably often confused, especially since they
are found in intimate association in nature. Spinels of other colours
are not very attractive to the eye, and are not likely to be in much
demand. Blue spinel (Plate XXVII, Fig. 4) is far from common, but the
shade is inclined to steely-blue, and is much inferior to the superb
tint of the true sapphire. Spinel is very hard and eminently suitable
for a ring-stone, but is seldom large and transparent enough for larger
articles of jewellery.

Spinel is an aluminate of magnesium corresponding to the formula
MgAl_{2}O_{4}, and therefore is closely akin to corundum, alumina, and
chrysoberyl, aluminate of beryllium. The composition may, however,
vary considerably owing to the isomorphous replacement of one element
by another; in particular, ferrous oxide or manganese oxide often
takes the place of some magnesia, and ferric oxide or chromic oxide is
found instead of part of the alumina. When pure, spinel is devoid of
colour, but such stones are exceedingly rare. No doubt chromic oxide is
responsible for the rose-red hue of balas-ruby, and also, when tempered
by ferric oxide, for the orange tint of rubicelle, and manganese is
probably the cause of the peculiar violet colour of almandine-spinel.
It is scarcely possible to define all the shades between blue and red
that may be assumed by spinel. Stones which are rich in iron are known
as pleonaste or ceylonite; they are quite opaque, but are sometimes
used for ornamental wear.

The name of the species comes from a diminutive form of σπῖνος, a
spark, and refers to the fiery red colour of the most valued kind
of spinel. It may be noted that the Latin equivalent of the word,
_carbunculus_, has been applied to the crimson garnet when cut
_en cabochon_. Balas is derived from _Balascia_, the old name for
Badakshan, the district from which the finest stones were brought in
mediæval times.

[Illustration: FIGS. 73, 74.—Spinel Crystals.]

Spinel, like diamond, belongs to the cubic system of crystalline
symmetry, and occurs in beautiful octahedra, or in flat
triangular-shaped plates (Figs. 73, 74) the girdles of which are
cleft at each corner, these plates being really twinned octahedra.
The refraction is, of course, single, and there is therefore no
double refraction or dichroism; this test furnishes the simplest
way of discriminating between the balas and the true ruby. Owing to
isomorphous replacement the value of the refractive index may lie
anywhere between 1·716 and 1·736. The lower values, about 1·720,
correspond to the most transparent red and blue stones; the deep
violet stones have values above 1·730. Spinel possesses little
colour-dispersion, or ‘fire.’ In the same way the values of the
specific gravity, even of the transparent stones, vary between 3·5
and 3·7, but the opaque ceylonite has values as high as 4·1. Spinel
is slightly softer than sapphire and ruby, and has the symbol 8 on
Mohs’s scale, and it is scarcely inferior in lustre to these stones.
Spinel is easily separated from garnet of similar colour by its lower
refractivity. Spinels run from 10s. to £5 a carat, depending on their
colour and quality, and exceptional stones command a higher rate.

Spinel always occurs in close association with corundum. The balas
and the true ruby are mixed together in the limestones of Burma and
Siam. Curiously enough, the spinel despite its lower hardness is found
in the river gravels in perfect crystals, whereas the rubies are
generally waterworn. Fine violet and blue spinels occur in the prolific
gem-gravels of Ceylon. A large waterworn octahedron and a rough mass,
both of a fine red colour, are exhibited in the Mineral Gallery of the
British Museum (Natural History), and a beautiful faceted blue stone is
shown close by.

The enormous red stone, oval in shape, which is set in front of the
English crown, is not a ruby, as it was formerly believed to be,
but a spinel. It was given to the gallant Black Prince by Pedro the
Cruel after the battle of Najera in 1367, and was subsequently worn
by Henry V upon his helmet at the battle of Agincourt. As usual with
Indian-fashioned stones it is pierced through the middle, but the hole
is now hidden by a small stone of similar colour.

The British Regalia also contains the famous stone called the Timur
Ruby or Khiraj-i-Alam (Tribute of the World), which weighs just over
352 carats, and is the largest spinel-ruby known. It is uncut, but
polished. Its history goes back to 1398, when it was captured by the
Amir Timur at Delhi. On the wane of the Tartar empire the stone became
the property of the Shahs of Persia, until it was given by Abbas I to
his friend and ally, the Mogul Emperor, Jehangir. It remained at Delhi
until, on the sack of that city by Nadir Shah in 1739, it, together
with immense booty, including the Koh-i-nor, fell into the hands of
the conqueror. Like the great diamond, it eventually came into the
possession of Runjit Singh at Lahore, and on the annexation of the
Punjab in 1850 passed to the East India Company. It was shown at the
Great Exhibition of 1851, and afterwards presented to Queen Victoria.

Mention has been made above (p. 121) of the blue spinel which is
manufactured in imitation of the true sapphire. The artificial stone is
quite different in tint from the blue spinel found in nature.

CHAPTER XXIII

GARNET

The important group of minerals which are known under the general name
of garnet provides an apt illustration of the fact that rarity is an
essential condition if a stone is to be accounted precious. Owing to
the large quantity of Bohemian garnets, of a not very attractive shade
of yellowish red, that have been literally poured upon the market
during the past half-century the species has become associated with
cheap and often ineffective jewellery, and has acquired a stigma which
completely prevents its attaining any popularity with those professing
a nice taste in gem-stones. It must not, however, be supposed that
garnet has entirely disappeared from high-class jewellery although the
name may not readily be found in a jeweller’s catalogue. Those whose
business it is to sell gem-stones are fully alive to the importance of
a name, and, as has already been remarked (p. 109), they have been fain
to meet the prejudices of their customers by offering garnets under
such misleading guises as ‘Cape-ruby,’ ‘Uralian emerald,’ or ‘olivine.’

Garnets may, moreover, figure under another name quite unintentionally.
Probably many a fine stone masquerades as a true ruby; the
impossibility of distinguishing these two species in certain cases by
eye alone is perhaps not widely recognized. An instructive instance
came under the writer’s notice a few years ago. A lady one day had
the misfortune to fracture one of the stones in a ruby ring that had
been in the possession of her family for upwards of a century, and
was originally purchased of a leading firm of jewellers in London.
She took the ring to her jeweller, and asked him to have the stone
replaced by another ruby. A day or two later he sent word that it was
scarcely worth while to put a ruby in because the stones in the ring
were paste. Naturally distressed at such an opinion of a ring which had
always been held in great esteem by her family, the lady consulted a
friend, who suggested showing it to the writer. A glance was sufficient
to prove that if the ring had been in use so long the stones could not
possibly be paste on account of the excellent state of their polish,
but a test with the refractometer showed that the stones were really
almandine-garnets, which so often closely resemble the true ruby in
appearance. Beautiful as the stones were, the ring was probably not
worth one-tenth what the value would have been had the stones been
rubies.

To the student of mineralogy garnet is for many reasons of peculiar
interest. It affords an excellent illustration of the facility which
certain elements possess for replacing one another without any great
disturbance of the crystalline form. Despite their apparent complexity
in composition all garnets conform to the same type of formula: lime,
magnesia, and ferrous and manganese oxides, and again alumina and
ferric and chromic oxides may replace each other in any proportion,
iron being present in two states of oxidation, and it would be rare
to find a stone which agrees in composition exactly with any of the
different varieties of garnet given below.

[Illustration: FIGS. 75, 76.—Garnet Crystals.]

Garnet belongs to the cubic system of crystalline symmetry.
Its crystals are commonly of two kinds, both of which are very
characteristic, the regular dodecahedron, _i.e._ twelve-faced figure
(Fig. 75), and the tetrakis-octahedron or three-faced octahedron (Fig.
76); the latter crystals are, especially when weather- or water-worn,
almost spherical in shape. Closer and more refined observations have
shown that garnet is seldom homogeneous, being usually composed of
several distinct individuals of a lower order of symmetry. Although
singly refractive as far as can be determined with the refractometer or
by deviation through a prism, yet when examined under the polarizing
microscope, garnets display invariably a small amount of local double
refraction. The transition from light to darkness is, however, not
sharp as in normal cases, but is prolonged into a kind of twilight.
In hardness, garnet is on the whole about the same as quartz, but
varies slightly; hessonite and andradite are a little softer, pyrope,
spessartite, and almandine are a little harder, while uvarovite is
almost the same. All the varieties except uvarovite are fusible when
heated before the blowpipe, and small fragments melt sufficiently on
the surface in the ordinary bunsen flame to adhere to the platinum
wire holding them. This test is very useful for separating rough red
garnets, pyrope or almandine, from red spinels or zircons of very
similar appearance. Far greater variation occurs in the other physical
characters. The specific gravity may have any value between 3·55 and
4·20, and the refractive index ranges between 1·740 and 1·890. Both the
specific gravity and the refractive index increase on the whole with
the percentage amount of iron.

Garnet is a prominent constituent of many kinds of rocks, but the
material most suitable for gem purposes occurs chiefly in crystalline
schists or metamorphic limestones. Pyrope and demantoid are furnished
by peridotites and the serpentines resulting from them; almandine and
spessartite come mostly from granites.

The name of the species is derived from the Latin _granatus_,
seed-like, and is suggested by the appearance of the spherical crystals
when embedded in their pudding-like matrix.

The varieties most adapted to jewellery are the fiery-red pyrope and
the crimson and columbine-red almandine; the closer they approach the
ruddy hue of ruby the better they are appreciated. Hessonite was at one
time in some demand, but it inclines too much to the yellowish shade
of red and possesses too little perfection of transparency to accord
with the taste of the present day. Demantoid provides beautiful, pale
and dark emerald-green stones, of brilliant lustre and high dispersion,
which are admirably adapted for use in pendants or necklaces; on
account of their comparative softness it would be unwise to risk them
in rings. In many stones the colour takes a yellowish shade, which
is less in demand. Uvarovite also occurs in attractive emerald-green
stones, but unfortunately none as yet have been found large enough for
cutting. A few truly magnificent spessartites are known—one, a splendid
example, weighing 6¾ carats, being in the possession of Sir Arthur
Church; but the species is far too seldom transparent to come into
general use. The price varies per carat from 2s. for common garnet to
10s. for stones most akin to ruby in colour, and exceptional demantoids
may realize even as much as £10 a carat. The old style of cutting was
almost invariably rounded or _en cabochon_, but at the present day the
brilliant-cut front and the step-cut back is most commonly adopted.

The several varieties will now be considered in detail.

(_a_) HESSONITE

(_Grossular_, _Cinnamon-Stone_, _Hyacinth_, _Jacinth_)

This variety, strictly a calcium-aluminium garnet corresponding to
the formula Ca_{3}Al_{2}(SiO_{4})_{3}, but generally containing some
ferric oxide and therefore tending towards andradite, is called by
several different names. In science it is usually termed grossular, a
word derived from _grossularia_, the botanical name for gooseberry, in
allusion to the colour and appearance of many crystals, or hessonite,
and less correctly essonite, words derived from the Greek ἥσσων
in reference to the inferior hardness of these stones as compared
with zircon of similar colour; in jewellery it is better known as
cinnamon-stone, if a golden-yellow in colour, or hyacinth or jacinth.
The last word, which is indiscriminately used for hessonite and yellow
zircon, but should more properly be applied to the latter, is derived
from an old Indian word (cf. p. 229); jewellers, however, retain it for
the garnet.

Only the yellow and orange shades of hessonite (Plate XXIX, Fig. 5)
are used for jewellery. Neither the brownish-green kind, to which the
term grossular may properly be applied, nor the rose-red is transparent
enough to serve as a gem-stone. Hessonite may mostly be recognized,
even when cut, by the curiously granular nature of its structure, just
as if it were composed of tiny grains imperfectly fused together; this
appearance, which is very characteristic, may readily be perceived if
the interior of the stone be viewed through a lens of moderate power.

The specific gravity varies from 3·55 to 3·66, and the refractive index
from 1·742 to 1·748. The hardness is on the whole slightly below that
of quartz. When heated before a blowpipe it easily fuses to a greenish
glass.

The most suitable material is found in some profusion in the
gem-gravels of Ceylon, in which it is mixed up with zircon of an almost
identical appearance; both are called hyacinth. Hessonites from other
localities, although attractive as museum specimens, are not large and
clear enough for cutting purposes. Switzerland at one time supplied
good stones, but the supply has long been exhausted.

(_b_) PYROPE

(‘_Cape-Ruby_’)

Often quite ruby-red in colour (Plate XXIX, Fig. 6), this variety
is probably the most popular of the garnets. It is strictly
a magnesium-aluminium garnet corresponding to the formula
Mg_{3}Al_{2}(SiO_{4})_{3}, but usually contains some ferrous oxide
and thus approaches almandine. Both are included among the precious
garnets. Its name is derived from πυρωπός, fire-like, in obvious
allusion to its characteristic colour.

Although at its best pyrope closely resembles ruby, its appearance is
often marred by a tinge of yellow which decidedly detracts from its
value. Pyrope generally passes as a variety of ruby, and under such
names as ‘Cape-ruby,’ ‘Arizona-ruby,’ depending on the origin of the
stones, commands a brisk sale. The specific gravity varies upwards
from 3·70, depending upon the percentage amount of iron present, and
similarly the refractive index varies upwards from 1·740; in the higher
values pyrope merges into almandine. Its hardness is slightly greater
than that of quartz, and may be expressed on Mohs’s scale by the symbol
7¼.

An enormous quantity of small red stones, mostly with a slight tinge of
yellow, have been brought to light at Teplitz, Aussig, and other spots
in the Bohemian Mittelgebirge, and a considerable industry in cutting
and marting them has grown up at Bilin. Fine ruby-red stones accompany
diamond in the ‘blue ground’ of the mines at Kimberley and also at the
Premier mine in the Transvaal. Similar stones are also found in Arizona
and Colorado in the United States, and in Australia, Rhodesia, and
elsewhere.

Although commonly quite small in size, pyrope has occasionally attained
to considerable size. According to De Boodt the Kaiser Rudolph II had
one in his possession valued at 45,000 thalers (about £6750). The
Imperial Treasury at Vienna contains a stone as large as a hen’s egg.
Another about the size of a pigeon’s egg is in the famous Green Vaults
at Dresden, and the King of Saxony has one, weighing 468½ carats, set
in an Order of the Golden Fleece.

(_c_) RHODOLITE

This charming pale-violet variety was found at Cowee Creek and at
Mason’s Branch, Macon County, North Carolina, U.S.A., but in too
limited amount to assume the position in jewellery it might otherwise
have expected. In composition it lies between pyrope and almandine,
and may be supposed to contain a proportion of two molecules of the
former to one of the latter. Its specific gravity is 3·84, refractive
index 1·760, and hardness 7¼. It exhibits in the spectroscope the
absorption-bands characteristic of almandine.

(_d_) ALMANDINE

(_Carbuncle_)

This variety is iron-aluminium garnet corresponding to the formula
Fe_{3}Al_{2}(SiO_{4})_{3}, but the composition is very variable. In
colour it is deep crimson and violet or columbine-red (Plate XXIX, Fig.
8), but with increasing percentage amount of ferric oxide it becomes
brown and black, and opaque, and quite unsuitable for jewellery. The
name of the variety is a corruption of Alabanda in Asia Minor, where
in Pliny’s time the best red stones were cut. Almandine is sometimes
known as Syriam, or incorrectly Syrian garnet, because at Syriam, once
the capital of the ancient kingdom of Pegu, which now forms part of
Lower Burma, such stones were cut and sold. Crimson stones, cut in the
familiar _en cabochon_ form and known as carbuncles, were extensively
employed for enriching metalwork, and a half-century or so ago were
very popular for ornamental wear, but their day has long since gone.
Such glowing stones are aptly described by their name, which is derived
from the Latin _carbunculus_, a little spark. In Pliny’s time, however,
the term was used indiscriminately for all red stones. It has already
been remarked that the word spinel has a similar significance.

The specific gravity varies from 3·90 for transparent stones to 4·20
for the densest black stones, and the refractive index may be as high
as 1·810. Almandine is one of the hardest of the garnets, and is
represented by the symbol 7½ on Mohs’s scale. The most interesting and
curious feature of almandine lies in the remarkable and characteristic
absorption-spectrum revealed when the transmitted light is examined
with a spectroscope (p. 61). The phenomenon is displayed most vividly
by the violet stones, and is, indeed, the cause of their peculiar
colour.

Although a common mineral, almandine of a quality fitted for jewellery
occurs in comparatively few localities. It is found in Ceylon, but
not so plentifully as hessonite. Good stones are mined in various
parts of India, and are nearly all cut at Delhi or Jaipur. Brazil
supplies good material, especially in the Minas Novas district of
Minas Geraes, where it accompanies topaz, and Uruguay also furnishes
serviceable stones. Almandine is found in Australia, and in many parts
of the United States. Recently small stones of good colour have been
discovered at Luisenfelde in German East Africa.

(_e_) SPESSARTITE

Properly a manganese-aluminium garnet corresponding to the formula
Mn_{3}Al_{2}(SiO_{4})_{3}, this variety generally contains iron in
both states of oxidation. If only transparent and large enough its
aurora-red colour would render it most acceptable in jewellery. Two
splendid stones have, indeed, been found in Ceylon (p. 211), and good
stones rather resembling hessonites have been quarried at Amelia
Court House in Virginia, and others have come from Nevada; otherwise,
spessartite is unknown as a gem-stone.

The specific gravity ranges from 4·0 to 4·3, and the refractive index
is about 1·81, both characters being high; the hardness is slightly
greater than that of quartz.

(_f_) ANDRADITE

(_Demantoid_, _Topazolite_, ‘_Olivine_’)

Andradite is strictly a calcium-iron garnet corresponding to the
formula Ca_{3}Fe_{2}(SiO_{4})_{3}, but as usual the composition varies
considerably. It is named after d’Andrada, a Portuguese mineralogist,
who made a study of garnet more than a century ago.

Once contemptuously styled common garnet, andradite suddenly sprang
into the rank of precious stones upon the discovery some thirty
years ago of the brilliant, green stones (Plate XXIX, Fig. 7) in the
serpentinous rock beside the Bobrovka stream, a tributary of the
Tschussowaja River, in the Sissersk district on the western side of
the Ural Mountains. The shade of green varies from olive through
pistachio to a pale emerald, and is probably due to chromic oxide. Its
brilliant lustre, almost challenging that of diamond, and its enormous
colour-dispersion, in which respect it actually transcends diamond,
raise it to a unique position among coloured stones. Unfortunately
its comparative softness limits it to such articles of jewellery as
pendants and necklaces, where it is not likely to be rubbed. When first
found it was supposed to be true emerald, which does actually occur
near Ekaterinburg, and was termed ‘Uralian emerald.’ When analysis
revealed its true nature, it received from science the slightly
inharmonious name of demantoid in compliment to its adamantine lustre.
Jewellers, however, prefer to designate it ‘olivine,’ not very happily,
because the stones usually cut are not olive-green and the name is
already in extensive use in science for a totally distinct species (p.
225); they recognized the hopelessness of endeavouring to find a market
for them as garnets. The yellow kind of andradite known as topazolite
would be an excellent gem-stone if only it were found large and
transparent enough. Ordinary andradite is brown or black, and opaque;
it has occasionally been used for mourning jewellery.

The specific gravity varies from 3·8 to 3·9, being about 3·85 for
demantoid, which has a high refractive index, varying from 1·880 to
1·890, and may with advantage be cut in the brilliant form. It is the
softest of the garnets, being only 6½ on Mohs’s scale.

(_g_) UVAROVITE

This variety, which is altogether unknown in jewellery, is
a calcium-iron garnet corresponding mainly to the formula
Ca_{3}Cr_{2}(SiO_{4})_{3}, but with some alumina always present, and
was named after a Russian minister. It has an attractive green colour,
and is, moreover, hard, being about on Mohs’s scale, but it has never
yet come to light of a size suitable for cutting. The specific gravity
is low, varying from 3·41 to 3·52. Unlike the kindred varieties it
cannot be fused by heating before an ordinary blowpipe.

CHAPTER XXIV

TOURMALINE

(_Rubellite_)

Tourmaline is unsurpassed even by corundum in variety of hue, and
it has during recent years rapidly advanced in public favour,
mainly owing to the prodigal profusion in which nature has formed
it in that favoured State, California, the garden of the west. Its
comparative softness militates against its use in rings, but its
gorgeous coloration renders it admirably fitted for service in any
article of jewellery, such as a brooch or a pendant, in which a large
central stone is required. Like all coloured stones it is generally
brilliant-cut in front and step-cut at the back, but occasionally it is
sufficiently fibrous in structure to display, when cut _en cabochon_,
pronounced chatoyancy.

The composition of this complex species has long been a vexed
question among mineralogists, but considerable light was
recently thrown on the subject by Schaller, who showed that all
varieties of tourmaline may be referred to a formula of the type
12SiO_{2}.3B_{2}O_{3}.(9-_x_)[(Al,Fe)_{2}O_{3}].3_x_[(Fe,Mn,Ca,
Mg,K_{2},Na_{2},Li_{2},H_{2})O].3H_{2}O. The ratios of boric oxide,
silica, and water are nearly constant in all analyses, but great
variation is possible in the proportions of the other constituents.
Having regard to this complexity, it is not surprising to find that
the range in colour is so great. Colourless stones, to which the name
achroite is sometimes given, were at one time exceedingly rare, but
they are now found in greater number in California. Stones which are
most suited to jewellery purposes are comparatively free from iron, and
apparently owe their wonderful tints to the alkaline earths; lithia,
for instance, is responsible for the beautiful tint of the highly
prized rubellite, and magnesia, no doubt, for the colour of the brown
stones of various tints. Tourmaline rich in iron is black and almost
opaque. It is a striking peculiarity of the species that the crystals
are rarely uniform in colour throughout, the boundaries between the
differently coloured portions being sharp and abrupt, and the tints
remarkably in contrast. Sometimes the sections are separated by planes
at right angles to the length of the crystal, and sometimes they are
zonal, bounded by cylindrical surfaces running parallel to the same
length. In the latter case a section perpendicular to the length shows
zones of at least three contrasting tints. In the Brazilian stones the
core is generally red, bounded by white, with green on the exterior,
while the reverse is the case in the Californian stones, the core being
green or yellow, bounded by white, with red on the exterior. Tourmaline
may, indeed, be found of almost every imaginable tint, except,
perhaps, the emerald green and the royal sapphire-blue. The principal
varieties are rose-red and pink (rubellite) (Plate XXVII, Fig. 1),
green (Brazilian emerald), indigo-blue (indicolite), blue (Brazilian
sapphire), yellowish green (Brazilian peridot) (Plate XXVII, Fig. 2),
honey-yellow (Ceylonese peridot), violet-red (siberite), and brown
(Plate XXVII, Fig. 8). The black, opaque stones are termed schorl.

The name of the species is derived from the Ceylonese word, _turamali_,
and was first employed when a parcel of gem-stones was brought to
Amsterdam from Ceylon in 1703; in Ceylon, however, the term is applied
by native jewellers to the yellow zircon commonly found in the island.
Schorl, the derivation of which is unknown, is the ancient name for the
species, and is still used in that sense by miners, but it has been
restricted by science to the black variety. The ‘Brazilian emerald’
was introduced into Europe in the seventeenth century and was not
favourably received, possibly because the stones were too dark in
colour and were not properly cut; that they should have been confused
with the true emerald is eloquent testimony to the extreme ignorance
of the characters of gem-stones prevalent in those dark ages. Achroite
comes from the Greek, ἄχροος, without colour.

[Illustration: FIG. 77.—Tourmaline Crystal.]

To the crystallographer tourmaline is one of the most interesting of
minerals. If the crystals, which are usually prismatic in form, are
doubly terminated, the development is so obviously different at the
two ends (Fig. 77) as to indicate that directional character in the
molecular arrangement, termed the polarity, which is borne out by other
physical properties. Tourmaline is remarkably dichroic, A brown stone,
except in very thin sections, is practically opaque to the ordinary
ray, and consequently a section cut parallel to the crystallographic
axis, _i.e._ to the length of a crystal prismatically developed,
transmits only the extraordinary ray. Such sections were in use for
yielding plane-polarized light before Nicol devised the calcite prism
known by his name (cf. p. 44). It is evident that tourmaline, unless
very light in tint, must be cut with the table facet parallel to that
axis, because otherwise the stone will appear dark and lifeless. The
values of the extraordinary and ordinary refractive indices range
between 1·614 and 1·638, and 1·633 and 1·669 respectively; the double
refraction, therefore, is fairly large, amounting to 0·025, and,
since the ordinary exceeds the extraordinary ray, its character is
negative. The specific gravity varies from 3·0 to 3·2. The lower values
in both characters correspond to the lighter coloured stones used in
jewellery; the black stones, as might be expected from their relative
richness in iron, are the densest. The hardness is only about the same
as that of quartz, or perhaps a little greater, varying from 7 to 7½.
It will be noticed that the range of refractivity overlaps that of
topaz (_q.v._) but the latter has a much smaller double refraction,
and may thus be distinguished (p. 29). Unmounted stones are still more
easily distinguished, because tourmaline floats in methylene iodide,
while topaz sinks. The pyro-electric phenomenon (cf. p. 82) for which
tourmaline is remarkable, although of little value as a test in the
case of a cut stone, is of great scientific interest, because it is
strong evidence of the peculiar crystalline symmetry pertaining to its
molecular arrangement. Tourmalines range in price from 5s. to 20s. a
carat according to their colour and quality, but exceptional stones may
command a higher rate.

Tourmaline is usually found in the pegmatite dykes of granites, but
it also occurs in schists and in crystalline limestones. Rubellite
is generally associated with the lithia mica, lepidolite; the groups
of delicate pink rubellite bespangling a background of greyish
white lepidolite are among the most beautiful of museum specimens.
Magnificent crystals of pink, blue, and green tourmaline have been
found in the neighbourhood of Ekaterinburg, principally at Mursinka, in
the Urals, Russia, and fine rubellite has come from the Urulga River,
and other spots near Nertschinsk, Transbaikal, Asiatic Russia. Elba
produces pink, yellowish, and green stones, frequently particoloured;
sometimes the crystals are blackened at the top, and are then known
locally as ‘nigger-heads.’ Ceylon supplies small yellow stones—the
original tourmaline—which are confused with the zircon of a similar
colour, and rubellite accompanies the ruby at Ava, Burma. Beautiful
crystals, green and red, often diversely coloured, come from various
parts, such as Minas Novas and Arassuahy, of the State of Minas Geraes,
Brazil. Suitable gem material has been found in numerous parts of
the United States. Paris and Hebron in Maine have produced gorgeous
pink and green crystals, and Auburn in the same state has supplied
deep-blue, green, and lilac stones. Fine crystals, mostly green, but
also pink and particoloured, occur in an albite quarry near the Conn
River at Haddam Neck, Connecticut. All former localities have, however,
been surpassed by the extraordinary abundance of superb green, and
especially pink, crystals at Pala and Mesa Grande in San Diego County,
California. As elsewhere, many-hued stones are common. The latter
locality supplies the more perfectly transparent crystals. Kunz states
that two remarkable rubellite crystals were found there, one being 45
mm. in length and 42 mm. in diameter, and the other 56 mm. in length
and 24 mm. in diameter. Madagascar, which has proved of recent years
to be rich in gem-stones, supplies green, yellow, and red stones, both
uniformly tinted and particoloured, which in beauty, though perhaps not
in size, bear comparison with any found elsewhere.

CHAPTER XXV

PERIDOT

The beautiful bottle-green stone, which from its delicate tint has
earned from appreciative admirers the poetical _sobriquet_ of the
evening emerald, and which has during recent years crept into popular
favour and now graces much of the more artistic jewellery, is named
as a gem-stone peridot—a word long in use among French jewellers, the
origin and meaning of which has been forgotten—but is known to science
either as olivine, on account of the olive-green colour sometimes
characterizing it, or as chrysolite. It is of interest to note that the
last word, derived from χρυσός, golden, and λίθος, stone, was in use at
the time of Pliny, but was employed for topaz and other yellow stones,
while his topaz, curiously enough, designated the modern peridot (cf.
p. 199), an inversion that has occurred in other words. The true
olivine must not be confused with the jewellers’ ‘olivine,’ which is a
green garnet from the Ural Mountains (p. 217). Peridot is comparatively
soft, the hardness varying from 6½ to 7 on Mohs’s scale, and is
suitable only for articles which are not likely to be scratched; the
polish of a peridot worn in a ring would soon deteriorate. The choicest
stones are in colour a lovely bottle-green (Plate XXIX, Fig. 2) of
various depths; the olive-green stones (Plate XXIX, Fig. 3) cannot
compare with their sisters in attractiveness. The step form of cutting
is considered the best for peridot, but it is sometimes cut round or
oval in shape, with brilliant-cut fronts.

Peridot is a silicate of magnesium and iron, corresponding to the
formula (Mg,Fe)_{2}SiO_{4}, ferrous iron, therefore, replacing
magnesia. To the ferrous iron it is indebted for its colour, the
pure magnesium silicate being almost colourless, and the olive tint
arises from the oxidation of the iron. The latitude in the composition
resulting from this replacement is evinced in the considerable range
that has been observed in the physical characters, but the crystalline
symmetry persists unaltered; the lower values correspond to the stones
that are usually met with as gems. Peridot belongs to the orthorhombic
system of crystalline symmetry, and the crystals, which display a
large number of faces, are prismatic in form and generally somewhat
flattened. The stones, however, that come into the market for cutting
as gems are rarely unbroken. The dichroism is rather faint, one of the
twin colours being slightly more yellowish than the other, but it is
more pronounced in the olive-tinted stones. The values of the least
and greatest of the principal indices of refraction vary greatly,
from 1·650 and 1·683 to 1·668 and 1·701, but the double refraction,
amounting to 0·033, remains unaffected. Peridot, though surpassed by
sphene in extent of double refraction, easily excels all the ordinary
gem-stones in this respect, and this character is readily recognizable
in a cut stone by the apparent doubling of the opposite edges when
viewed through the table facet (cf. p. 41). An equally large
variation occurs in the specific gravity, namely, from 3·3 to 3·5.

[Illustration: _PLATE XXVII_

1. RUBELLITE
2. TOURMALINE
3. BALAS-RUBY
4. BLUE SPINEL
5. QUARTZ
6. WHITE OPAL
7. AMETHYST
8. TOURMALINE
9. BLACK OPAL
10. FIRE OPAL
11. ALEXANDRITE (_By daylight_)
12. CHRYSOBERYL
13. ALEXANDRITE (_By artificial light_)

GEM-STONES]

Peridots of deep bottle-green hue command moderate prices at the
present day, about 30s. a carat being asked for large stones; the paler
tinted stones run down to a few shillings a carat. The rate per carat
may be very much larger for stones of exceptional size and quality.

Olivine, to use the ordinary mineralogical term, is a common and
important constituent of certain kinds of igneous rocks, and it is
also found in those strange bodies, meteorites, which come to us
from outer cosmical space. Except in basaltic lavas, it occurs in
grains and rarely in well-shaped crystals. Stones that are large and
transparent enough for cutting purposes come almost entirely from the
island Zebirget or St. John situated on the west coast of the Red Sea,
opposite to the port of Berenice. This island belongs to the Khedive of
Egypt, and is at present leased to a French syndicate. It is believed
to be the same as the mysterious island which produced the ‘topaz’
of Pliny’s time. Magnificent stones have been discovered here, rich
green in colour, and 20 to 30, and occasionally as much as 80, carats
in weight when cut; a rough mass attained to the large weight of 190
carats. Pretty, light-green stones are supplied by Queensland, and
peridots of a less pleasing dark-yellowish shade of green, and without
any sign of crystal form, have during recent years come from North
America. Stones rather similar to those from Queensland have latterly
been found in the Bernardino Valley in Upper Burma, not far from the
ruby mines.

CHAPTER XXVI

ZIRCON

(_Jargoon_, _Hyacinth_, _Jacinth_)

Zircon, which, if known at all in jewellery, is called by its variety
names, jargoon and hyacinth or jacinth, is a species that deserves
greater recognition than it receives. The colourless stones rival
even diamond in splendour of brilliance and display of ‘fire’; the
leaf-green stones (Plate XXIX, Fig. 13) possess a restful beauty
that commends itself; the deep-red stones (Plate XXIX, Fig. 14), if
somewhat sombre, have a certain grandeur; and no other species produces
such magnificent stones of golden-yellow hue (Plate XXIX, Fig. 12).
Zircon is well known in Ceylon, which supplies the world with the
finest specimens, and is highly appreciated by the inhabitants of that
sunny isle, but it scarcely finds a place in jewellery elsewhere. The
colourless stones are cut as brilliants, but brilliant-cut fronts with
step-cut backs is the usual style adopted for the coloured stones.

Zircon is a silicate of zirconium corresponding to the formula
ZrSiO_{4}, but uranium and the rare earths are generally present in
small quantities. The aurora-red variety is known as hyacinth or
jacinth, and the term jargoon is applied to the other transparent
varieties, and especially to the yellow stones. The most attractive
colours shown by zircon are leaf-green, golden-yellow, and deep red.
Other common colours are brown, greenish, and sky-blue. Colourless
stones are not found in nature, but result from the application of heat
to the yellow and brown stones.

The name of the species is ancient, and comes from the Arabic _zarqūn_,
vermilion, or the Persian _zarqūn_, gold-coloured. From the same
source in all probability is derived the word jargoon through the
French _jargon_ and the Italian _giacone_. Hyacinth (cf. p. 211) is
transliterated from the Greek ὑάκινθος, itself adapted from an old
Indian word; it is in no way connected with the flower of the same
name. The last word has seen some changes of meaning. In Pliny’s time
yellow zircons were indiscriminately classified with other yellow
stones as chrysolite. His hyacinth was used for the sapphire of the
present day, but was subsequently applied to any transparent corundum.
Upon the introduction of the terms, sapphire and ruby, for the blue and
the red corundum hyacinth became restricted to the other varieties, of
which the yellow was the commonest. In the darkness of the Middle Ages
it was loosely employed for all yellow stones emanating from India,
and was finally, with increasing discernment in the characters of
gem-stones, assigned to the yellow zircon, since it was the commonest
yellow stone from India.

[Illustration: FIG. 78.—Zircon Crystal.]

Considered from the scientific point of view, zircon is by far the most
interesting and the most remarkable of the gem-stones. The problem
presented by its characters and constitution is one that still awaits
a satisfactory solution. Certain zircons, which are found as rolled
pebbles in Ceylon and never show any trace of crystalline faces, have
very nearly single refraction, and the values of the refractive index
vary from 1·790 to 1·840, and the specific gravity is about 4·00 to
4·14, and the hardness is slightly greater than that of quartz, being
about 7¼. On the other hand, such stones as the red zircons from
Expailly have remarkably different properties. They show crystalline
faces with tetragonal symmetry, the faces present being four prismatic
faces mutually intersecting at right angles and four inclined faces at
each end (Fig. 78). They have large double refraction, varying from
0·044 to 0·062, which is readily discerned in a cut stone (cf. p.
41), and the refractive indices are high, the ordinary index varying
from 1·923 to 1·931 and the extraordinary from 1·967 to 1·993. Since
the ordinary is less than the extraordinary index the sign of the
double refraction is positive. The specific gravity likewise is much
higher, varying from 4·67 to 4·71. The second type, therefore, sinks
in molten silver-thallium nitrate, whereas the first type floats. The
second type is also slightly harder, being about 7½ on Mohs’s scale.
By heating either of these types the physical characters are not much
altered, except that the colour is weakened or entirely driven off and
some change takes place in the double refraction. But between these
two types may be found zircons upon which the effect of heating is
striking. They seem to contract in size so that the specific gravity
increases as much as three units in the first place of decimals, and a
corresponding increase takes place in the refractive indices, and in
the amount of double refraction. The cause of these changes remains a
matter of speculation. Evidently a third type of zircon exists which
is capable of most intimate association with either of the other
types, and which is very susceptible to the effect of heat. It may be
noted that stones of the intermediate type are usually characterized
by a banded or zonal structure suggesting a want of homogeneity. The
theory has been advanced that zircon contains an unknown element which
has not yet been separated from zirconium. Zircon of the first type
favours green, sky-blue, and golden-yellow colours; honey-yellow, light
green, blue, and red colours characterize the second type; and the
intermediate stones are mostly yellowish green, cloudy blue, and green.

It is another peculiarity of zircon that it sometimes shows in the
spectroscope absorption bands (p. 61), which were observed in 1866 by
Church. Many zircons do not exhibit the bands at all, and others only
display the two prominent bands in the red end of the spectrum.

Of all the gem-stones zircon alone approaches diamond in brilliance of
lustre, and it also possesses considerable ‘fire’; it can, of course,
be readily distinguished by its inferior hardness, but a judgment based
merely on inspection by eye might easily be erroneous.

According to Church, who has made a lifelong study of zircon, the green
and yellowish stones of the first variety emit a brilliant orange light
when being ground on a copper wheel charged with diamond dust, and
the golden stones of the intermediate type glow with a fine orange
incandescence in the flame of a bunsen burner; the latter phenomenon is
supposed to be due to the presence of thoria.

The leaf-green stones almost invariably show a series of parallel bands
in the interior.

Zircons vary from 5s. to 15s. a carat, but exceptional stones may be
worth more.

By far the finest stones come from Ceylon. The colourless stones are
there known as ‘Matura diamonds,’ and the hyacinth includes garnet
(hessonite) of similar colour, which is found with it in the same
gravels. The stones are always water-worn. Small hyacinths and deep-red
stones come from Expailly, Auvergne, France, and yellowish-red crystals
are found in the Ilmen Mountains, Orenburg, Russia. Remarkably fine
red stones have been discovered at Mudgee, New South Wales, and
yellowish-brown stones accompany diamond at the Kimberley mines, South
Africa.

CHAPTER XXVII

CHRYSOBERYL

(_Chrysolite_, _Cat’s-Eye_, _Cymophane_, _Alexandrite_)

Chrysoberyl has at times enjoyed fleeting popularity on account of the
excellent cat’s-eyes cut from the fibrous stones, and in the form of
alexandrite it meets with a steadier, if still limited, demand. It is a
gem-stone that is seldom met with in ordinary jewellery, although its
considerable hardness befits it for all such purposes.

Chrysoberyl is in composition an aluminate of beryllium corresponding
to the formula BeAl_{2}O_{4}, and is therefore closely akin to spinel.
It usually contains some ferric and chromic oxides in place of alumina,
and ferrous oxide in place of beryllia, and it is to these accessory
constituents that its tints are due. Other gem-stones containing the
uncommon element beryllium are phenakite and beryl. Pale yellowish
green, the commonest colour, is supposed to be caused by ferrous
oxide; such stones are known to jewellers as chrysolite (Plate XXVII,
Fig. 12). Cat’s-eyes (Plate XXIX, Fig. 1) have often also a brownish
shade of green. The bluish green and dark olive-green stones known
as alexandrite (Plate XXVII, Figs. 11, 13) differ in appearance so
markedly from their fairer sisters that their common parentage seems
almost incredible. The dull fires that glow within them, and the
curious change that comes over them at night, add a touch of mystery to
these dark stones. Chromic oxide is held responsible for their colour.
The cat’s-eyes are, of course, always cut _en cabochon_, but otherwise
chrysoberyl is faceted.

The name of the species is composed of two Greek words, χρυσός, golden,
and βήρυλλος, beryl, and etymologically more correctly defines the
lighter-coloured stones, which were, indeed, at one time the only kind
known. Chrysolite from χρυσὁς, golden, and λίθος, stone, has much the
same significance. This name is preferred by jewellers, but in science
it is applied to an entirely different species, which is known in
jewellery as peridot. Cymophane, from κῦμα, wave, and φαίνειν, appear,
refers to the peculiar opalescence characteristic of cat’s-eyes; it
is sometimes used to designate these stones, but does not find a
place within the vocabulary of jewellery. Alexandrite is named after
Alexander II, Czar of Russia, because it first came to light on his
birthday. That circumstance, coupled with its display of the national
colours, green and red, and its at one time restriction to the mining
district near Ekaterinburg, renders it dear to the heart of all loyal
Russians.

Chrysoberyl crystallizes in the orthorhombic system, and occurs in
rather dull, complex crystals, which are sometimes so remarkably
twinned, especially in the variety called alexandrite, as to simulate
hexagonal crystals. In keeping with the crystalline symmetry it
is doubly refractive and biaxial, having two directions of single
refraction. The least and the greatest of the principal indices of
refraction may have any values between 1·742 and 1·749, and 1·750 and
1·757, respectively, the maximum amount of double refraction remaining
always the same, namely, 0·009. The mean principal refractive index
is close to the least; the sign of the double refraction is therefore
positive, and the shadow-edge corresponding to the lower index, as
seen in the refractometer, has little, if any, perceptible motion
when the stone is rotated. The converse is the case with corundum;
the sign is negative, and it is the shadow-edge corresponding to the
greater refractive index that remains unaltered in position on rotation
of the stone. This test would suffice to separate chrysoberyl from
yellow corundum, even if the refractive indices of the former were
not sensibly lower than those of the latter. Also, the dichroism of
chrysolite is stronger than that of yellow sapphires. In alexandrite
this phenomenon is most prominent; the absorptive tints, columbine-red,
orange, and emerald-green, corresponding to the three principal
optical directions, are in striking contrast, and the first differs
so much from the intrinsic colour of the stone as to be obvious to
the unaided eye, and is the cause of the red tints visible in a cut
stone. The curious change in colour of alexandrite, from leaf-green to
raspberry-red, that takes place when the stone is seen by artificial
light, is due to a different cause, as has been pointed out above (p.
54). The effect is illustrated by Figs. 11, 13 on Plate XXVII, which
represent a fine Ceylon stone as seen by daylight and artificial light;
the influence of dichroism may be noticed in the former picture. The
specific gravity of chrysoberyl varies from 3·68 to 3·78. In hardness
this species ranks above spinel and comes next to corundum, being
given the symbol 8½ on Mohs’s scale. Certain stones contain a multitude
of microscopic channels arranged in parallel position. When the stones
are cut with their rounded surface parallel to the channels, a broadish
band of light is visible running across the stone at right angles
to them, and suggests the pupil of a cat’s eye, whence the common
name for the stones. The fact that the channels are hollow causes an
opalescence, which is absent from the quartz cat’s-eye.

The most important locality for the yellowish chrysoberyl is the rich
district of Minas Novas, Minas Geraes, Brazil, where it occurs in the
form of pebbles, and excellent material is also supplied by Ceylon,
in both crystals and rounded pebbles. Other places for chrysolite are
Haddam, Connecticut, and Greenfield, Saratoga County, New York, in
the United States, and recently in the gem-gravels near the Somabula
Forest, Rhodesia. Ceylon supplies some of the best cat’s-eyes.
Alexandrite was first discovered, as already stated, at the emerald
mines near Ekaterinburg, in the Urals; but the supply is now nearly
exhausted. A poorer quality comes from Takowaja, also in the Urals.
Good alexandrite has come to light in Ceylon, and most of the stones
that are placed on the market at the present day have emanated from
that island. The Ceylon stones reach a considerable size, often as much
as from 10 to 20 carats in weight; the Russian stones have a better
colour and are more beautiful, but they are less transparent, and
rarely exceed a carat in weight. Good chrysolite may be worth from 10s.
to £2 a carat, and cat’s-eye runs from £1 to £4 a carat, depending
upon the quality. Alexandrites meet with a steady demand in Russia, and
fine stones are scarce; flawless stones about a carat in weight are
worth as much as £30 a carat, and even quite ordinary stones fetch £4 a
carat.

From Ceylon, that interesting home of gems, have originated some
magnificent chrysoberyls, including a superb chrysolite, 80¾ carats
in weight, and another, a splendid brownish yellow in colour and
very even in tint, and two large alexandrites, green in daylight and
a rich red by night, weighing 63⅜ and 28-23/32 carats. The finest
cut chrysolite existing is probably the one exhibited in the Mineral
Gallery of the British Museum (Natural History). Absolutely flawless
and weighing 43¾ carats, it was formerly contained in the famous Hope
collection, and is described on page 56 and figured on Plate XXI of
the catalogue prepared by B. Hertz, which was published in 1839; the
weight there given includes the brilliants and the ring in which it
was mounted. It is shown, about actual size, in Plate XXVII, Fig. 12.
A magnificent cat’s-eye, 35·5 by 35 mm. in size, which also formed
part of the Hope collection, was included in the crown jewels taken
from the King of Kandy in 1815. The crystalline markings in the cut
stone are so arranged that the lower half shows an altar overhung by a
torch. The stone has been famous in Ceylon for many ages. It was set
in gold with rubies cut _en cabochon_. Two fine Ceylon alexandrites of
exceptional merit, weighing 42 and 26¾ carats, are also exhibited in
the Mineral Gallery of the British Museum (Natural History). The former
is illustrated in Plate XXVII, Figs. 11, 13, as seen in daylight and in
artificial light.

CHAPTER XXVIII

QUARTZ

(_Rock-Crystal_, _Amethyst_, _Citrine_, _Cairngorm_, _Cat’s-Eye_,
_Tiger’s-Eye_)

Although the commonest and, in its natural form, the most easily
recognizable of mineral substances, quartz nevertheless holds a not
inconspicuous position among gem-stones, because, as amethyst (Plate
XXVII, Fig. 7), it provides stones of the finest violet colour;
moreover, the yellow quartz (Plate XXVII, Fig. 5) so ably vies with the
true topaz that it is universally known to jewellers by the name of the
latter species, and is too often confounded with it, and the lustrous,
limpid rock-crystal even aspires to the local title of ‘diamond.’
For all purposes where a violet or yellow stone is required, quartz
is admirably suited; it is hard and durable, and it has the merit,
or possibly to some minds the drawback, of being moderate in price.
Despite its comparative lack of ‘fire,’ rock-crystal might replace
paste in rings and buckles with considerable advantage from the point
of view of durability. The chatoyant quartz, especially in the form
known as tiger’s-eye, will for beauty bear comparison with the true
cat’s-eye, which is a variety of chrysoberyl. Except that cat’s-eye is
cut _en cabochon_, quartz is step- or sometimes brilliant-cut.

Ranking with corundum next to diamond as the simplest in composition
of the gem-stones, quartz is the crystallized form of silica, oxide
of silicon, corresponding to the formula SiO_{2}. When pure, it
is entirely devoid of the faintest trace of colour and absolutely
water-clear. Such stones are called rock-crystal, and it is easy to
understand why in early days it was supposed to represent a form
of petrified water. It is these brilliant, transparent stones that
are, when small, known in many localities as ‘diamonds.’ Before
the manufacture of glass was discovered and brought to perfection,
rock-crystal was in considerable use for fashioning into cups, vases,
and so forth. The beautiful tints characterizing quartz are due to the
usual metallic oxides. To manganese is given the credit of the superb
purple or violet colour of amethyst, which varies considerably in
depth. Jewellers are inclined to distinguish the deep-coloured stones
with the prefix ‘oriental,’ but the practice is to be deprecated, since
it might lead to confusion with the true oriental amethyst, which is
a purple sapphire, one of the rarest varieties of corundum. Quartz
of a yellow hue is properly called citrine, but, as already stated,
jewellers habitually prefer the name ‘topaz’ for it, and distinguish
the true topaz by the prefix Brazilian—not a very happy term, since
both the yellow topaz and the yellow quartz occur plentifully in
Brazil. Sometimes the yellow quartz is termed occidental, Spanish,
or false topaz. Stones with a brownish or smoky tinge of yellow are
called cairngorm, or Scotch topaz. The colour of the yellow stones
is doubtless due to a trace of ferric oxide. Stones of a smoky brown
colour are known as smoky-quartz. Rose-quartz, which is rose-red or
pink in colour and hazy in texture, is comparatively rare; strange
to say, it has never been found in distinct crystals. The tint,
which may be due to titanium, is fugitive, and fades on exposure to
strong sunlight. In milky quartz, as the name suggests, the interior
is so hazy as to impart to the stone a milky appearance. It has
frequently happened that quartz has crystallized after the formation
of other minerals, with the result that the latter are found inside
it. Prase, or mother-of-emerald, which at one time was supposed to
be the mother-rock of emerald, is a quartz coloured leek-green by
actinolite fibres in the interior. Specimens containing hair-like
fibres of rutile—the so-called _flêches d’amour_—are common in mineral
collections, and are sometimes to be seen worked. When enclosing a
massive, light-coloured, fibrous mineral, the stones have a chatoyant
effect, and display, when suitably cut, a fine cat’s-eye effect; in
tiger’s-eye the enclosed mineral is crocidolite, an asbestos, the
original blue hue of which has been changed to a fine golden-brown
by oxidation. Quartz which contains scales of mica, hematite, or
other flaky mineral has a vivid spangled appearance, and is known
as aventurine; it has occasionally been employed for brooches or
similar articles of jewellery. Rainbow-quartz, or iris, is a quartz
which contains cracks, the chromatic effect being the result of the
interference of light reflected from them; it has been artificially
produced by heating the stone and suddenly cooling it.

The name of the species is an old German mining term of unknown
meaning which has been in general use in all languages since the
sixteenth century. Amethyst is derived from ἀμέθυστος, not drunken,
possibly from a foolish notion that the wearer was exempt from the
usual consequences of unrestrained libations. Pliny suggests as an
alternative explanation that its colour approximates to, but does not
quite reach, that of wine. Aventurine, from _aventura_, an accident,
was first applied to glass spangled with copper, the effect being
said to have been accidentally discovered owing to a number of copper
filings falling into a pot of molten glass in a Venetian factory.

[Illustration: FIG. 79.—Quartz Crystal.]

Quartz belongs to the hexagonal system of crystalline symmetry,
and crystallizes in the familiar six-sided prisms terminated by
six inclined, often triangular, faces (Fig. 79); twins are common,
though they are not always obvious from the outward development. In
accordance with the symmetry the refraction is double, and there is one
direction of single refraction, namely, that parallel to the edge of
the prism. The ordinary refractive index has the value 1·544, and the
extraordinary 1·553, and since the latter is the greater, the sign of
the double refraction is positive. The double refraction is small in
amount, but is large enough to enable the apparent doubling of certain
of the opposite edges of a faceted stone to be perceptible when viewed
with a lens through the table-facet. The dichroism of the deep-coloured
stones is quite distinct. Quartz has only about the same amount of
colour dispersion as ordinary glass, and lacks, therefore, ‘fire.’
The application of strong heat tends, as usual, to weaken or drive
off the colour. Thus the dense smoky-quartz found in Spain, Brazil,
and elsewhere is converted into stones of a colour varying from light
yellow to reddish brown according to the amount and duration of the
application. In the case of amethyst the colour is changed to a deep
orange, or entirely driven off if the temperature be high enough. Its
density is very constant, varying only from 2·654 to 2·660; the purest
stones are the lightest. To it has been assigned the symbol 7 on Mohs’s
scale of hardness.

To physicists quartz is one of the most interesting of minerals because
of its power of rotating, to an extent depending upon the thickness of
the section, the plane of polarization of a beam of light traversing it
in a direction parallel to the prism edge. It appears, moreover, from
a study of the pyro-electric and general physical characters, that its
molecular structure has a helical arrangement, which, like all screws,
may have a right- or left-handed character. Amethyst is, in fact,
invariably composed of separate twin individuals, alternately right-
and left-handed; in some remarkable crystals the section at right
angles to the prism edge is composed of triangular sectors, alternately
of different hands and of different tints—purple and white. To the
twinning is due the rippled fracture and the feathery inclusions so
characteristic of amethyst.

Besides its use for ornamental purposes, quartz finds a place as the
material for lenses intended for delicate photographic work, because
its transparency to the ultra-violet light is so much greater than
that of glass. Spectacle lenses made of it are in demand, because they
are not liable to scratches, and retain, therefore, their polish
indefinitely. When fused in the oxyhydrogen flame, quartz becomes a
silica glass, of specific gravity 2·2 and hardness 5 on Mohs’s scale,
which has proved of great service for laboratory ware, because it
withstands sudden and unequal heating without any danger of fracture;
it has also in fine threads been invaluable for delicate torsion work,
because it acquires not the smallest amount of permanent twist, in this
respect being superior to the finest silk threads.

Clear rock-crystal fetches little more than the cost of the cutting;
citrine and amethyst are worth from 1s. to 5s. a carat, depending
upon the quality and size of the stone; smoky-quartz is practically
valueless; rose-quartz realizes less than 1s. a carat; and the value
of cat’s-eye is also small—only 1s. to 2s. 6d. a carat. Tiger’s-eye at
one time commanded as much as 25s. a carat, but the supply exceeded the
demand, with the consequent collapse in the price.

Beautiful, brilliant, and limpid rock-crystal is found in various parts
of the world: in the Swiss Alps, at Bourg d’Oisans in the Dauphiné
Alps, France, in the famous Carrara marble, in the Marmaros Comitat
of Hungary, and in the United States, Brazil, Madagascar, and Japan.
Small lustrous stones, known in their localities as ‘Isle of Wight,’
‘Cornish,’ or ‘Bristol diamonds,’ are found in our own country. Brazil
supplies stones out of which have been cut the clear balls used in
crystal-gazing. The finest amethysts come from Brazil—especially the
State of Rio Grande do Sul—and from Uruguay, India, and the gem-gravels
of Ceylon; good stones also occur at Ekaterinburg, in the Ural
Mountains. A splendid Brazilian amethyst, weighing 334 carats, and
two Russian stones—one hexagonal in contour, weighing 88 carats, and
the other, a deep purple in colour with a circular table, weighing 73
carats—are exhibited in the British Museum (Natural History). Cairngorm
is known from the place of that name in Banffshire, Scotland, whence
fine specimens have emanated; it is a gem much valued in that country.
Fine cairngorm has also originated from Pike’s Peak, Colorado. Splendid
yellow stones have had their birth in the States of Minas Geraes, São
Paulo, and Goyãz, of Brazil—especially in the last. The fine Spanish
smoky-quartz, which, as already stated, turns yellow on heating, comes
from Hinojosa, in the Province of Cordova. The delicate rose-quartz
is known at Bodenmais in Bavaria, Paris in Maine, United States, and
Ekaterinburg in the Ural Mountains. The finest cat’s-eyes are found in
India and Ceylon, and are high in favour with the natives. Greenish
stones of an inferior quality are brought from the Fichtelgebirge in
Bavaria, and are sold as ‘Hungarian cat’s-eyes,’ despite the fact
that no such stone occurs in Hungary—another instance of jewellers’
disdain for accuracy. Tiger’s-eye occurs in considerable quantity in
the neighbourhood of Griquatown, Griqualand West, South Africa. A
silicified crocidolite, in which the blue colour is retained, comes
also from Salzburg, and is known as sapphire- or azure-quartz, or
siderite.

Certain of the pebbles found on the seashore of our coasts, especially
off the Isle of Wight and North Wales, cut into attractive, clear
stones, more or less yellow in colour; but examples suitable for the
purpose are not so numerous as might be supposed, and do not reward any
casual search. _Les affaires sont les affaires._ The local lapidary,
instead of explaining that the pebbles brought to him are not worth
cutting, finds it more convenient and profitable to substitute for
them other, inferior and badly cut, stones, bought by the gross, or
even paste stones; the customer, on the other hand, is contented with
a pretty bauble, and is not grateful for the information that it might
have been obtained for a fraction of the sum paid.

CHAPTER XXIX

CHALCEDONY, AGATE, ETC.

Chalcedony and agate, and their endless varieties, are composed
mainly of silica, but the separate individual crystals are so small
as to be invisible to the unaided eyesight, and occasionally are
so extremely minute that the structure is almost amorphous. The
colour and appearance vary greatly, depending upon the impurities
contained in the stone, and, since both have been made a criterion for
differentiation of types, a host of names have come into use, none of
which are susceptible of strict definition. On the whole, these stones
may be divided into two groups: chalcedony, in which the structure is
concretionary and the colour comparatively uniform, and agate, in which
the arrangement takes the form of bands, varying greatly in tint and
colour.

The refraction, though double in the individual, is irregular over the
stone as a whole, and the indices approximate to 1·550. The specific
gravity ranges from 2·62 to 2·64, depending upon the impurities
present. The degree of hardness is about the same as that of quartz,
namely, 7 on Mohs’s scale. All kinds are more or less porous, and
stones of a dull colour are therefore artificially tinted after being
worked.

The term chalcedony, derived from χαλκηδών the name of a town in
Asia Minor, is usually confined to stones of a greyish tinge. Stones
artificially coloured an emerald green have been cut and put upon
the market as ‘emeraldine.’ Carnelian is a clear red chalcedony,
and sard is somewhat similar, but brownish in tint. Chrysoprase is
apple-green in colour, nickel oxide being supposed to be the agent.
Prase (cf. p. 240), which is a dull leek-green in hue, may also in
part be referred here; the name comes from πράσμον, a leek. Plasma,
which may have the same derivation, is a brighter leek-green. Jasper
is a chalcedony coloured blood-red by iron oxide, while bloodstone is
a green chalcedony spotted with jasper; they are popular stones for
signet rings. Flint, an opaque chalcedony, breaks with a sharp cutting
edge, and was much in request with early man as a tool or a weapon; its
property of giving sparks when struck with steel rendered it invaluable
before the invention of matches. Hornstone is somewhat similar, but
more brittle, while chert is a flinty rock.

Agate, named after the river Achates in Sicily, where it was found at
the time of Theophrastus, has a peculiar banded structure, the bands
being usually irregular in shape, following the configuration of the
cavity in which it was formed. Moss-agate, or mocha-stone, contains
moss-like inclusions of some fibrous mineral. Onyx is an agate with
regular bands, the layers having sharply different colours; when black
and white, it has, in days gone by, been employed for cameos. Sardonyx
is similar in structure, but red and white in colour. Agate is used in
delicate balances for supporting the steel knife-edges of the balance
itself and of the panholders, and is largely employed—especially when
artificially coloured—for umbrella handles and similar articles.

Chalcedony and agate are found the whole world over, but India, and
particularly Brazil, are noted for their fine carnelians and agates.

CHAPTER XXX

OPAL

(_White Opal_, _Black Opal_, _Fire-Opal_)

That opal in early times excited keen admiration is evident from
Pliny’s enthusiastic description of these stones: “For in them you
shall see the burning fire of the carbuncle, the glorious purple of the
amethyst, the green sea of the emerald, all glittering together in an
incredible mixture of light.” During much of last century, owing to the
foolish superstition that ill-luck dogs the footsteps of the wearer,
the species lay under a cloud, which has even now not quite dispersed,
but exercises a prejudicial effect upon the fortunes of the stone. It
has, however, recently attracted considerable attention owing to the
discovery of the splendid black opals in Australia; at one moment black
with the darkness of night, at the next by a chance movement glowing
with vivid crimson flame, such stones may justly be considered the most
remarkable in modern jewellery. At the present day opal is divided by
jewellers roughly into two main groups: ‘white’ (Plate XXVII, Fig. 6)
and ‘black’ (Plate XXVII, Fig. 9), according as the tint is light or
dark, fire-opal (Plate XXVII, Fig. 10) standing in a separate category.

Opal differs from the rest of the principal gem-stones in being
not a crystalline body, but a solidified jelly, and it depends for
its attractiveness upon the characteristic play of colour, known,
in consequence, as opalescence (cf. p. 39), which arises from a
peculiarity in the structure. Opal is mainly silica, SiO_{2}, in
composition, but contains in addition an amount of water varying in
precious opal from 6 to 10 per cent. As the original jelly cooled, it
became riddled throughout with cracks, which were afterwards generally
filled with opal matter, containing a different amount of water,
and therefore differing slightly in refractivity from the original
substance. The structure not being quite homogeneous, each crack has
the same action upon light as a soap-film, and gives rise to precisely
similar phenomena; the thinner and more uniform the cracks, the greater
the splendour of the chromatic display, the particular tint depending
upon the direction in which the stone is viewed. The cracks in certain
opals were not filled up, and therefore contain air. Such stones appear
opaque and devoid of opalescence until plunged into water; they are
consequently known as hydrophane, from ὕδωρ, water, and φαίνεσθαι, to
make appear. Owing to the effect of total-reflection, light was stopped
on the hither side of the cracks before they were filled with water,
which is not far inferior to opal in refractivity; it is surprising how
much water these stones will absorb.

Opal is colourless when pure, but is nearly always more or less milky
and opaque, or tinted various dull shades by ferric oxide, magnesia
or, alumina. The so-called black opal is generally a dark grey or
blue, and very rarely quite black. That the coloration is not due
to ordinary absorption, but to the action of cracks in the stone,
is shown by the fact that the transmitted light is complementary to
the reflected light; the blue opal is, for instance, a yellow when
held up so that light has passed through it. In many black opals the
opalescent material occurs in far too tiny pieces to be cut separately,
and the whole iron-stained matrix is cut and polished and sold under
the name ‘opal-matrix.’ The reddish and orange-coloured stones known
as fire-opal have pronounced colour and only slight milkiness; they
display the customary opalescence in certain directions. These stones
are often faceted, but otherwise opals are cut _en cabochon_, either
flat or steep—generally the former in brooches and pendants, and the
latter in rings. Opal is somewhat soft, varying from 5 to 6½ on Mohs’s
scale, and is therefore easily scratched. The specific gravity ranges
from 2·10 to 2·20, and the refractive index from 1·444 to 1·464, the
refraction, of course, being always single. It is unwise to immerse
opals in liquids on account of their porosity.

The name opal comes to us through the Latin _opallus_, which was used
for the same species as understood by the term at the present day, but
the word has a far older origin, which has not been traced. The Romans
also called the mineral _pæderos_, the Greek form of Cupid, a name
applied to all rosy stones. The name cacholong, for the bluish-white
porcelain variety, which is very porous and adheres to the tongue, is
of Tartar origin; the stone is highly valued in the East.

The oldest mines, which up to quite a recent date were the only
extensive deposit of opal known, were at Cserwenitsa, near Kashau,
in Hungary. From them in all probability emanated the opals known to
the Romans. The opals from this locality were generally quite small,
and large pieces were rare and commanded high prices. The Hungary
mines, however, proved quite unable to compete with the rich fields
at White Cliffs, New South Wales, in spite of the efforts that were
made to depreciate and exclude from the market the new stones, and at
the present time few of the opals on the market come from them. As so
often happens, the White Cliffs deposit was discovered by accident.
In 1889 a hunter, when tracking a wounded kangaroo, chanced to pick
up an attractively coloured opal. The district is so waterless and
forbidding that, but for such a chance, the opals might have long lain
hidden. They occur in seams in deposits of Cretaceous Age in a variety
of ways, filling cavities in rocks or sandstones, or cracks in wood, or
replacing wood, saurian bones, and some spiky mineral, which may have
been glauberite. In recent years, another rich deposit was discovered
farther north, on both sides of the boundary between Queensland and
New South Wales. The field is remarkable for the darkness of its
opals, which are called ‘black opal’ in contradistinction to the
lighter-coloured stones previously known. From Lightning Ridge in
New South Wales come stones stained deep black which quite merit the
designation black opal. The sandstone in which they are found is
rich in iron, and this is no doubt responsible for the deepness of
their tint. Mexico is noted for the fire-opal, which is found at
Esperanza, Queretaro, and Zimapan; but other kinds of opal also are
found at these places.

[Illustration: _PLATE XXVIII_

OPAL MINES, WHITE CLIFFS, NEW SOUTH WALES]

The price of opal varies greatly, according to the intrinsic colour
and the uniformity and brilliance of the opalescence. Common opal can
be bought at as low a rate as 1s. a carat, while black opal ranges
from 10s. to £8 a carat; but a good dark stone displaying a flaming
opalescence commands a fancy figure, fine stones of this class being
exceedingly rare. Fire-opal enjoys only a limited popularity now,
though a few years ago it was in some demand; the price runs from 2s.
to 10s. a carat.

CHAPTER XXXI

FELSPAR

(_Moonstone, Sunstone, Labradorite, Amazon-Stone_)

Though second to none among minerals in scientific interest, whether
regarded from the point of view of their crystalline characters or
the important part they play in the formation of rocks, the group
included under the general name felspar occupies but a humble place
in jewellery. It consists of three distinct species, orthoclase,
albite, and anorthite, which are silicates of aluminium, and potassium,
sodium, or calcium, corresponding to the formulae KAlSi_{3}O_{8},
NaAlSi_{3}O_{8}, and CaAl_{2}Si_{2}O_{8} respectively, and also of
species intermediate in composition between albite and orthoclase, or
albite and anorthite. While differing in crystalline symmetry, all
are characterized by two directions of cleavage which are nearly at
right angles to one another. The double refraction, which is slight in
amount, is biaxial in character and variable in sign. The values of the
least and greatest of the indices of refraction range between 1·52 and
1·53, and 1·53 and 1·55 respectively, the double refraction at the same
time varying from 0·007 to 0·012. The specific gravity lies between
2·48 and 2·66, and the hardness ranges between the degrees 6 and 7 on
Mohs’s scale.

Moonstone (Plate XXIX, Fig. 4), which is mainly pure orthoclase, alone
is at all common in jewellery. It forms such an admirable contrasting
frame for large coloured stones that it deserves greater popularity;
no doubt the cheapness of the stones militates against their proper
appreciation. The milky, bluish opalescence from which they take their
name is caused by the reflection of light at the thin twin-lamellæ
of which the structure is composed. They are always cut more or less
steeply _en cabochon_. The finest stones were at one time cut from
the felspar that came from the St. Gothard district in Switzerland
and was in consequence known as adularia from the neighbouring Adular
Mountains, somewhat incorrectly, since none occurs at the latter
locality. At the present day practically all the moonstones on the
market come from Ceylon. They run in price from £3 to £20 per oz. (28
grams).

Sunstone is a felspar containing flakes of hematite or goethite which
impart a spangled bronze appearance to the stones. Good material occurs
in parts of Norway. The remarkable sheen of labradorite or blue felspar
has its origin in the interference of light at lamellar surfaces in
the interior; the uniformity of the colour over comparatively large
areas testifies to the regularity of the lamellar arrangement. The
finest specimens were brought from the Isle of St. Paul off the coast
of Labrador, where they were first discovered in 1770; large masses
also occur on the coast itself. Amazon-stone is an opaque green felspar
which occurs in the Ilmen Mountains, Orenburg, Russia, and at Pike’s
Peak, Colorado, United States. It obtains its name from the Amazon
River, where, however, none has ever been found; there may have been
some confusion with a jade or similar stone.

Occasionally clear colourless felspar has been faceted, and then
closely resembles rock-crystal. A careful determination of the
refractive indices and the specific gravity serves to discriminate
between them.

[Illustration: _PLATE XXIX_

1. CAT’S EYE
2. PERIDOT
3. PERIDOT
4. MOONSTONE
5. HESSONITE
6. PYROPE
7. DEMANTOID
8. ALMANDINE
9. SPODUMENE
10. KUNZITE
11. HIDDENITE
12. ZIRCON
13. ZIRCON
14. ZIRCON
15. ANDALUSITE
16. NEPHRITE
17. TURQUOISE
18. JADEITE

GEM-STONES]

CHAPTER XXXII

TURQUOISE, ODONTOLITE, VARISCITE

Of all the opaque stones turquoise (Plate XXIX, Fig. 17) alone finds a
prominent place in jewellery and can aspire to rank with the precious
stones. The colour varies from a sky-blue or a greenish blue to a
yellowish green or apple-green. Only the former tints, which are at
the same time the rarer, are in general demand, and they possess the
great advantage of harmonizing with the tint of the gold setting.
The blue colours are, especially in the case of the Siberian stones,
by no means permanent, and fade in course of time. Turquoise is
amorphous and seldom crystalline, and is therefore somewhat porous; it
should consequently never be immersed in liquids or be contaminated
with greasy and dirty matter lest the dreaded change of colour be
brought about. The stones are translucent in thin sections, and a
good observation is possible with the refractometer if the back of
the stone is flat and polished, since only the section immediately
adjacent to the instrument is concerned; the refractive index is about
1·61. The specific gravity varies from 2·75 to 2·89. Turquoise has a
hardness of slightly under 6 on Mohs’s scale, and takes a good polish,
which is fairly durable, since on account of the comparative opacity
of the stones scratches on the surface are not very noticeable.
In composition it is a complex phosphate of aluminium and copper,
corresponding to the formula CuOH.[6Al(OH)_{2}].H{5}.(PO_{4})_{4},
with ferric oxide replacing some alumina. The blue colour is due to
the copper constituent, and the predominance of iron may cause the
greenish shades; but the water contained in the stones plays no mean
part, since they turn a dirty green when it is driven off. The faded
colour can sometimes be restored by immersion of the stone in ammonia
and subsequent application of grease, but the effect is not lasting.
Attempts are sometimes made to improve inferior stones by impregnating
them with Berlin blue, but with only qualified success. Turquoises are
said to be affected by the perspiration from the skin.

The name of the species comes from a French word meaning Turkish, and
arises from the fact that the gem-stone first reached Europe by way of
Turkey. Another, but less obvious, suggestion is that it is derived
from the Persian name for the species, _piruzeh_. Our turquoise and
other phosphates of similar appearance were probably known to Pliny
under the three names _callais_, _callaina_, and _callaica_.

The finest turquoise still comes from the famous mines near Nishapur in
the Persian province of Khorassan, where it was known in very ancient
times; it is found with limonite filling the cracks and cavities
in a brecciated porphyritic trachyte. Pieces of the turquoise and
limonite from here are sometimes cut without removal of the latter,
and sold as ‘turquoise-matrix,’ when the precious stones are too tiny
to be worth separate working. It also occurs at Serbâl in the Sinai
Peninsula. Among the more recent localities may be mentioned Los
Cerillos Mountains, New Mexico; Sierra Nevada, Nevada, where pale blue
and green stones are found; San Bernardino County, California, where
again the stones are rather pale; and Arizona, where it occurs in pale
greenish-blue stones.

Some of the stones that have been seen are not the true turquoise but
odontolite, or bone turquoise, which consists of the teeth and bones
of mastodon or other extinct animals, phosphate of iron being the
colouring material. These stones may easily be recognized by their
organic structure, which is clearly visible if viewed with a strong
lens or under the microscope. Moreover, odontolite invariably contains
some calcium carbonate, and effervescence takes place if it be touched
with hydrochloric acid. Turquoise dissolves in hydrochloric acid, but
without effervescence, and since it contains copper, a fine blue colour
is imparted to the solution by the addition of ammonia. Odontolite has
a higher specific gravity, 3·0 to 3·5, but lower hardness, 5 on Mohs’s
scale.

Variscite, the hydrated phosphate of aluminium, corresponding to the
formula AlPO_{4} + 2H_{2}O, is found in masses resembling a greenish
turquoise, but it is much softer, being only 4 on Mohs’s scale. The
specific gravity is 2·55. Round nodular masses of variscite are found
in Utah.

CHAPTER XXXIII

JADE

Though not usually accounted precious among European nations or in
Western civilization in general, jade was held in extraordinary
esteem by primitive man, and was fashioned by him into ornaments and
utensils, often of considerable beauty, and even at the present day
it ranks among the Chinese and Japanese peoples above all precious
stones; indeed, the Chinese word _Yu_ and the Japanese words _Giyuku_
or _Tama_ signify both jade and precious stones in general. According
to the Chinese, jade is the prototype of all gems, and unites in
itself the five cardinal virtues—_Jin_, charity; _Gi_, modesty; _Yu_,
courage; _Ketsu_, justice; and _Chi_, wisdom. When powdered and mixed
with water, it is supposed to be a powerful remedy for all kinds of
internal disorders, to strengthen the frame and prevent fatigue, to
prolong life, and, if taken in sufficient quantity just before death,
to prevent decomposition.

Jade is a general term that includes properly two distinct mineral
species, nephrite or greenstone, and jadeite, which are very similar
in appearance, both being fibrous and tough in texture, and more or
less greenish in colour; but it is also applied to other species such
as saussurite, californite, bowenite, and plasma, which have somewhat
similar characters. The word jade is a corruption of the Spanish
_pietra di hijada_, kidney-stone, in allusion to its supposed efficacy
in diseases of that organ.

Nephrite or greenstone (Plate XXIX, Fig. 16) is the commoner of
the two jades. It is closely allied to the mineral hornblende, a
silicate of magnesium, iron, and calcium corresponding to the formula
Ca(Mg,Fe)_{3}(SiO_{3})_{4}, the magnesia being replaceable by ferrous
oxide. Microscopic examination shows that the structure consists of
innumerable independent fibres foliated or matted together, the former
character giving rise to a slaty and the latter to a horny appearance
in the stone as seen by the unaided eye. The colour varies from grey
to leaf- and dark-green, the tint deepening as the relative amount of
iron in the composition increases, and brown tints result from the
oxidation of the iron along cracks in the stone. The hardness is 6½ on
Mohs’s scale; nephrite is therefore about as hard as ordinary glass and
softer than quartz. When polished, it always acquires a greasy lustre.
The specific gravity ranges from 2·9 to 3·1. The least and greatest of
the principal refractive indices are 1·606 and 1·632 respectively, the
double refraction being biaxial and negative; the coloured fibres also
display dichroism. All these differential effects are, however, masked
in the stone because of the irregularity of the aggregation. Nephrite
is fusible before the blowpipe, but only with difficulty. Its name is
derived from the Greek word νεφρός, kidney, the allusion being the same
as for jade.

Many of the prehistoric implements found in Mexico and in the Swiss
Lake Habitations are composed of nephrite, but it is uncertain where
the mineral was obtained. Much of the material used by the Chinese
at the present time comes from spots near the southern boundary of
Eastern Turkestan, especially in the valleys of the rivers Karakash and
Yarkand in the Kwen Lun range of mountains; it is also found farther
north at the river Kashgar. It occurs in various provinces of China,
namely, Shensi, Kwei Chau, Kwang Tung, Yunnan, and Manchuria. Gigantic
waterworn boulders have been found in the Government of Irkutsk, near
Lake Baikal, in eastern Siberia, the first discovery being made in the
bed of the Onot stream by the explorer and prospector J. P. Alibert,
in 1850. A large boulder of this kind, weighing over half a ton (1156
lb., or 524·5 kg.), is exhibited in the Mineral Gallery of the British
Museum (Natural History). An enormous mass, weighing over 2 tons (4718
lb., or 2140 kg.), was discovered at Jordansmühl, Silesia, by Dr.
G. F. Kunz, and is now in the magnificent collection of jade formed
by Mr. Heber R. Bishop. Beautiful greenstone occurs in New Zealand,
particularly in the Middle Island. The Maoris have long used it for
various useful and ornamental purposes, the most common being indicated
by their general name for the species, _punamu_, axe-stone; _kawakawa_
is the ordinary green variety, a fine section of which is shown on the
wall of the Mineral Gallery of the British Museum (Natural History),
while _inanga_, a grey variety, and _kahurangi_, a pale-green and
translucent variety, are rare and highly prized.

Jadeite (Plate XXIX, Fig. 18) is by far the rarer of the two jades,
and is the choicest gem with the Chinese. In composition it is a
silicate of sodium and aluminium with the formula NaAl(SiO_{3})_{2},
corresponding to the lithium mineral spodumene (p. 265). It has the
same toughness and greasy lustre as nephrite, but is harder, being
represented by the symbol 7 on Mohs’s scale, and thus only slightly,
if at all, softer than quartz. The other characters are also higher;
the specific gravity is about 3·34, and the least and greatest of the
principal refractive indices are 1·66 and 1·68, the double refraction
being biaxial and negative. The colour varies from white to almost
an emerald green, the latter being especially prized, and often the
green colour runs in streaks through the white. Jadeite fuses readily
before the blowpipe to blebby glass, more easily than is the case with
nephrite.

The finest jadeite comes from the Mogaung district in Upper Burma,
where it is found in boulders and also with albite in dykes in a
dark-green serpentine. The export trade to China, which absorbs
practically the whole of the output, is exceedingly valuable, and
realizes nearly as much as the produce of the ruby mines. Jadeite is
also found in the Shensi and Yunnan provinces of China, and in Tibet.

• • • • •

A few words may be said about the other jade-like minerals. Saussurite,
which is named after H. B. de Saussure, has resulted from the
decomposition of a felspar, and is nearly akin to the mineral zoisite.
It has the customary toughness of structure, and is greenish grey to
white in colour. Its specific gravity is about 3·2, and hardness 6½ to
7 on Mohs’s scale. It occurs near Lake Geneva. Bowenite is a green
serpentine (p. 289) which is found at Smithfield, Rhode Island, U.S.A.,
and in New Zealand and Afghanistan. Californite and plasma are compact
varieties of idocrase (p. 275) and chalcedony (p. 247) respectively.
Verdite is a stone of rich green colour which is found in the form of
large boulders in the North Kaap River, South Africa; it is composed of
green mica (fuchsite) and some clayey matter.

Jade has of recent years been imitated in glass, but the latter is
recognizable by its vitreous lustre and inferior hardness, and sooner
or later by its frangibility.

CHAPTER XXXIV

SPODUMENE, IOLITE, BENITOITE

SPODUMENE

(_Kunzite_, _Hiddenite_)

Till a few years ago scarcely known outside the ranks of mineralogists,
spodumene suddenly leaped into notice in 1903 upon the discovery of the
lovely lilac-coloured stones (Plate XXIX, Fig. 10) at Pala, San Diego
County, California; they shortly afterwards received the name kunzite
after the well-known expert in gems, Dr. G. F. Kunz. The stones were
found here in a pegmatite dyke, and were of all shades, ranging from
pale pink to deep lilac, and at times as much as 150 carats in weight.
Paler kunzite occurs with beryl and tourmaline at Coahuila Mountain
in Riverside County, California, and colourless stones have recently
come to light in Madagascar. Kunzite is remarkable for its wonderful
dichroism; the beautiful violet tint that springs out in one direction
comes with greater surprise because of the uninteresting yellowish
tints in other directions. Unlike spodumene in general, kunzite is
phosphorescent under the influence of radium.

The emerald-green variety (Plate XXIX, Fig. 11), named hiddenite after
Mr. W. E. Hidden, who discovered in 1881 the only known occurrence, in
Alexander County, North Carolina, would no doubt have become popular
had the supply of material not been so very limited; few stones were
found, and the variety has never come to light elsewhere. The colour is
supposed to be due to chromic acid. Hiddenite being also dichroic, the
tint varies with the direction.

Spodumene is ordinarily rather a pale yellowish in hue, and, as its
name (which is derived from σποδίος, ash-coloured) suggests, is not
very attractive. Clear, lemon-yellow stones (Plate XXIX, Fig. 9) are
found in Brazil and Madagascar.

The species is interesting scientifically because it contains the
rare element lithium; it is a silicate of aluminium and lithium,
corresponding to the formula LiAl(SiO_{3})_{2}. The double refraction
is biaxial in character and positive in sign, the least and greatest
of the refractive indices being 1·660 and 1·675; the specific gravity
is 3·185, and hardness 6½ to 7 on Mohs’s scale. Spodumene has an easy
cleavage, and the cut stones call therefore for careful handling, lest
they be flawed or fractured. Two faceted stones, a beautiful kunzite
and a fine hiddenite, weighing 60 and 2½ carats respectively, are
exhibited in the British Museum (Natural History).

IOLITE

Known also by various other names—cordierite, dichroite, and
water-sapphire (_saphire d’eau_)—this species owes its interest
to the remarkable dichroism characterizing it, the principal
colours—smoky-blue and yellowish white—being in such contrast as to
be obvious to the unaided eye. The stones that are usually worked have
intrinsically a smoky-blue colour, and are found in waterworn masses
in the river-gravels of Ceylon, whence is the origin of the name
water-sapphire. Iolite, from ἴον, violet, and λίθος, stone, refers to
the colour; cordierite is named after Cordier, a French geologist, who
first studied the crystallography of the species; and dichroite, of
course, alludes to the most prominent character of the species.

Iolite is a silicate of aluminium and of magnesium and iron
corresponding to the formula H_{2}(Mg,Fe)_{4}Al_{8}Si_{10}O_{37}.
The double refraction is small in amount, biaxial in character, and
negative in sign, the least and greatest of the refractive indices
being 1·543 and 1·551; the specific gravity is 2·63, and hardness 7 on
Mohs’s scale. Iolite, if used, is worked and polished; it is seldom
faceted. A large worked piece, weighing 177 grams, which was formerly
in the Hawkins Collection, is exhibited in the British Museum (Natural
History).

BENITOITE

The babe among gem-stones, benitoite first saw the light of day a few
years ago, early in 1907. It occurs with the rare mineral neptunite,
which was previously known only from Greenland, in narrow veins of
natrolite in Diablo Range near the head-waters of the San Benito River,
San Benito County, California. Despite careful search the species has
not been found except within the original restricted area. To science
it is interesting both because of its composition, a silico-titanate
of barium, corresponding to the formula BaTiSi_{3}O_{9}, and because
its crystals belong to a class of crystalline symmetry which has
hitherto not been represented among minerals. The double refraction
is uniaxial, and since the ordinary index of refraction is 1·757 and
the extraordinary 1·804, it is positive in sign and large in amount,
namely, 0·047. The stones are characterized by strong dichroism, the
colour corresponding to the ordinary ray being white, and to the
extraordinary greenish blue to indigo depending upon the tint of the
stone. To obtain the best effect the stone must therefore be cut with
the table-facet parallel to the crystallographic axis. The specific
gravity is 3·65, and hardness 6½ on Mohs’s scale. When first discovered
the species was supposed to be sapphire, and many stones were cut and
sold as such. It is, however, much softer than sapphire, and is readily
distinguished by its optical characters, since it possesses greater
double refraction and of differing sign, so that, when tested with the
refractometer, the shadow-edge corresponding to the lower index of
refraction remains fixed in the case of benitoite, whereas the contrary
happens with sapphire. Benitoite also, unlike sapphire, fuses easily
to a transparent glass. Its blue colour, which is supposed to be due
to a small amount of free titanic acid present, appears to be stable.
Several stones as large as 1½ to 2 carats in weight have been found.
The largest of all, perfectly flawless, weighs just over 7 carats, and
is remarkable because it is about three times the next largest in point
of weight; it is the property of Mr. G. Eacret, of San Francisco.

CHAPTER XXXV

EUCLASE, PHENAKITE, BERYLLONITE

EUCLASE

This species comes near beryl in chemical composition, being a
silicate of aluminium and beryllium corresponding to the formula
Be(AlOH)SiO_{4}, and closely resembles aquamarine in colour and
appearance when cut. Owing to the rarity of the mineral good specimens
command high prices for museum collections, and it is seldom worth
while cutting it for jewellery. It derives its name from its easy
cleavage, εὖ easily, and κλάσις fracture. The double refraction is
biaxial in character and positive in sign, the least and greatest of
the refractive indices being 1·651 and 1·670 respectively; the specific
gravity is 3·07, and the hardness 7½ on Mohs’s scale. The colour is
usually a sea-green, but sometimes blue. Euclase occurs with topaz at
the rich mineral district of Minas Novas, Minas Geraes, Brazil, and has
also been found in the Ural district, Russia.

PHENAKITE

Another beryllium mineral, phenakite owes its name to the frequency
with which it has been mistaken for quartz, being derived from φέναξ,
deceiver. The clear, colourless crystals, somewhat complex in form,
have at times been cut, but they lack ‘fire,’ and despite their
brilliant lustre meet with little demand. The composition is a silicate
of beryllium corresponding to the formula Be_{2}SiO_{4}. The double
refraction is uniaxial, and since the ordinary, 1·652, is less than
the extraordinary index, 1·667, it is positive in sign; the specific
gravity is 2·99, and the hardness is almost equal to that of topaz,
being about 7½ to 8 on Mohs’s scale.

Fine stones have long been known near Ekaterinburg in the Ural
Mountains, and have recently been discovered in Brazil.

BERYLLONITE

As its name suggests, this mineral also contains beryllium, being
a soda phosphate corresponding to the formula NaBePO_{4}. Clear,
colourless stones, which occur at Stoneham, Maine, U.S.A., have been
cut, but the lack of ‘fire,’ the easy cleavage, and comparative
softness, the symbol being 5½ on Mohs’s scale, unfit it for use in
jewellery. The double refraction is biaxial in character and negative
in sign, the least and the greatest of the refractive indices being
1·553 and 1·565 respectively.

CHAPTER XXXVI

ENSTATITE, DIOPSIDE, KYANITE, ANDALUSITE, IDOCRASE, EPIDOTE,
SPHENE, AXINITE, PREHNITE, APATITE, DIOPTASE

ENSTATITE

(‘_Green Garnet_’)

The small green stones which accompany the diamond in South Africa have
been cut and put on the market as ‘green garnet.’ They are, however,
in no way connected with garnet, but belong to a mineral species
called enstatite, which is a silicate of magnesium corresponding to
the formula MgSiO_{3}; the green colour is due to a small amount of
ferrous oxide which replaces magnesia. The double refraction is biaxial
in character and positive in sign, the least and greatest of the
refractive indices being 1·665 and 1·674 respectively; the specific
gravity ranges from 3·10 to 3·13, and the hardness is only about 5½
on Mohs’s scale. The dichroism is perceptible, the twin-colours being
yellowish and green, and, as usual, is more pronounced the deeper the
colour of the stone. There is also a good cleavage in two different
directions.

With increasing percentage amount of iron enstatite passes into
hypersthene. The colour becomes a dark brownish green, and an increase
takes place in the physical constants, the least and greatest of the
refractive indices attaining to 1·692 and 1·705 respectively, and
the specific gravity ranging from 3·4 to 3·5. Hypersthene is never
sufficiently transparent for faceting, but when spangled with small
scales of brookite it is sometimes cut _en cabochon_.

The name enstatite is derived from ἐνστάτης, an opponent, referring to
the infusibility of the mineral before the blowpipe, and hypersthene
comes from ὑπερσθένος, very tough.

An altered enstatite, leek-green in colour and with nearly the
composition of serpentine (p. 289), has been cut _en cabochon_. It has
much lower specific gravity, only 2·6, and lower hardness, 3½ to 4 on
Mohs’s scale. It is named bastite from Baste in the Harz Mountains,
where it was first discovered.

DIOPSIDE

This species, which is also known as malacolite and alalite, provides
stones of a leaf-green colour which have occasionally been cut. It
is a silicate of calcium and magnesium corresponding to the formula
MgCa(SiO_{3})_{2}, but usually contains in place of magnesia some
ferrous oxide, to which it owes its colour; with increase in the
percentage amount of iron the colour deepens and the physical constants
change. The double refraction is large in amount, 0·028, biaxial
in character, and positive in sign. The least and greatest of the
refractive indices corresponding to the stones suitable for jewellery
range about 1·671 and 1·699 respectively, but they may be as high as
1·732 and 1·750 in the two cases. The specific gravity varies from 3·20
to 3·38, and the hardness from 5 to 6 on Mohs’s scale. Dichroism is
noticeable in deep-coloured stones, but is not very marked.

The name diopside comes from δίς, double, and ὄψις, appearance, in
allusion to the effect resulting from the double refraction; malacolite
is derived from μαλακός, soft, because the mineral is softer than the
felspar associated with it; and alalite is named after the principal
locality, Ala Valley, Piedmont, Italy.

KYANITE

Kyanite, also known as disthene, is interesting for two reasons. Its
structure is so grained in character that the hardness varies in the
same stone from 5 to 7 on Mohs’s scale; it can therefore be scratched
by a knife in some directions, but not in others (p. 79). It has the
same chemical composition as andalusite, both being silicates of
aluminium corresponding to the formula Al_{2}SiO_{5}, but possesses
very different physical characters, a fact which shows how large
a share the molecular grouping has in determining the aspect of
crystallized substances. It is biaxial with small negative double
refraction, the least and greatest of the refractive indices being
1·72 and 1·73 respectively; the specific gravity is 3·61. It occurs in
sky-blue prismatic crystals, whitish at the edges, in schist near St.
Gothard, Switzerland. It is seldom cut.

Kyanite is derived from its colour, κύανος blue, and disthene, from its
variable hardness, δίς, twice, and σθένος, strong.

ANDALUSITE

Andalusite bears no resemblance whatever to kyanite, although, as has
been stated above, the composition of the two species is essentially
the same. It is usually light bottle-green in colour, and more rarely
brown and reddish. Its extreme dichroism is its most remarkable
character, the twin colours being olive-green and red. The reddish
gleams that are reflected from the interior are in sharp contrast with
the general colour of the stone, and impart to it a weird effect (Plate
XXIX, Fig. 15). Cut stones are often confused with tourmalines, and
can, indeed, only be distinguished from the latter with certainty by
noting on the refractometer the smaller amount of double refraction
and the difference in its character. The least and greatest of the
refractive indices are 1·62 and 1·643 respectively, and the double
refraction, 0·011, about half that of tourmaline, is biaxial and
negative; the specific gravity is 3·18, and hardness 7½ on Mohs’s scale.

Good stones are found at Minas Novas, Minas Geraes, Brazil, and in
the gem-gravels of Ceylon. It was first known from the province of
Andalusia, Spain, whence is the origin of its name.

IDOCRASE

(_Vesuvianite_, _Californite_)

Idocrase, also known as vesuvianite, is occasionally found in the
form of transparent, leaf-green, and yellowish-brown stones which,
when cut, may be mistaken for diopside and epidote respectively, but
are distinguishable from both by the extreme smallness of their
double refraction. Californite is a compact variety which has all the
appearances of a jade; its colour is green, or nearly colourless with
green streaks.

In composition idocrase is a silicate of aluminium and calcium, the
precise formula of which is uncertain, but may be—

(Ca,Mn,Mg,Fe)_{2}[(Al,Fe)(OH,F)]Si_{2}O_{7}.

The double refraction, which is uniaxial in character and negative in
sign, may be less than 0·001, and never exceeds 0·006, so that it is
not easily detected with the refractometer, even in sodium light. The
refractive indices vary enormously in value, from 1·702 to 1·726 for
the ordinary, and from 1·706 to 1·732 for the extraordinary ray. The
specific gravity varies from 3·35 to 3·45, and the hardness is about 6½
on Mohs’s scale.

The name idocrase, from εἴδος, form, and κρᾶσις, mixture, was assigned
to the species by Haüy, but his reasons have little meaning at the
present day. The other names are taken from the localities where the
species and the variety were first discovered.

Bright, green crystals come from Russia, and also from Ala Valley,
Piedmont, and Mount Vesuvius, Italy. Californite is found in large
masses in Siskiyon and Fresno Counties, California.

EPIDOTE

(_Pistacite_)

Epidote often possesses a peculiar shade of yellowish green, similar
to that of the pistachio-nut—hence the origin of its alternative
name—which is unique among minerals, though scarcely pleasing enough
to recommend it to general taste. Its ready cleavage renders it liable
to flaws; nevertheless, it is occasionally faceted. The name epidote,
from ἐπίδοσις, increase, was given to it by Haüy, but not on very
precise crystallographical grounds.

In composition this species is a silicate of calcium and aluminium,
with some ferric oxide in place of alumina, corresponding to the
complex formula, Ca_{2}(Al,Fe)_{2}[(Al,Fe)OH](SiO_{4})_{3}. It
occurs in monoclinic, prismatic crystals richly endowed with natural
faces. The colour deepens with increase in the percentage amount of
iron, and the stones become almost opaque. The double refraction is
large in amount, 0·031, biaxial in character, and negative in sign.
The dichroism is conspicuous in transparent stones, the twin-tints
corresponding to the principal optical directions being green, brown,
and yellow. The values of the least and greatest of the refractive
indices given by transparent stones are 1·735 and 1·766 respectively;
the specific gravity varies from 3·25 to 3·50, and the hardness from 6
to 7 on Mohs’s scale.

Transparent crystals have come from Knappenwand, Untersulzbachtal,
Salzburg, Austria; Traversella, Piedmont, Italy; and Arendal, Nedenäs,
Norway. Magnificent, but very dark, crystals were discovered about ten
years ago on Prince of Wales Island, Alaska.

SPHENE

(_Titanite_)

The clear, green, yellow, or brownish stones provided by this species
would be welcomed in jewellery because of their brilliant and
almost adamantine lustre, but, unfortunately, they are too soft to
withstand much wear, the hardness being only 5½ on Mohs’s scale. In
composition sphene is a silico-titanate of calcium corresponding to
the formula CaTiSiO_{5}, and in this respect comes near the recently
discovered gem-stone, benitoite. The refractive indices lie outside
the range of the refractometer, the values of the least and the
greatest of the refractive indices varying from 1·888 and 1·917 to
1·914 and 2·053 respectively. It is to this high refraction that it
owes its brilliant lustre. The double refraction, which is biaxial in
character and positive in sign, is so large that the apparent doubling
of the opposite edges of a cut stone when viewed through one of the
faces is obvious to the unaided eye (cf. p. 41). Cut stones have
additional interest on account of the vivid dichroism displayed, the
twin-tints, colourless, yellow, and reddish yellow, corresponding to
the three principal optical directions, being in strong contrast. The
specific gravity ranges from 3·35 to 3·45. The negative test with the
refractometer (cf p. 26), the softness, and the large amount of double
refraction suffice to distinguish this species from gem-stones of
similar appearance.

The name sphene, from σφήν, wedge, alludes to the shape of the natural
crystals. The alternative name is obviously due to the fact that the
species contains titanium.

Good stones have come from the St. Gothard district, Switzerland.

AXINITE

Called axinite from the shape of its crystals—ἀξίνη, axe—this species
supplies small, clear, clove-brown, honey-yellow, and violet stones
which can be cut for those who care for a stone out of the ordinary.
The composition is a boro-silicate of aluminium and calcium, with
varying amounts of iron and manganese, corresponding to the formula
(Ca,Fe)_{3}Al_{2}(B.OH)Si_{4}O_{15}. Axinite is interesting on
account of its strong dichroism, the twin-tints corresponding to the
principal optical directions being violet, brown, and green. The double
refraction is biaxial in character and negative in sign, the least and
greatest of the refractive indices being 1·674 and 1·684; the specific
gravity is 3·28, and hardness about 6½ to 7, or rather under that of
quartz.

The best examples have been found at St. Cristophe, Bourg d’Oisans, in
the Dauphiné, France. Violet axinite is a novelty that has come within
recent years from Rosebery, Montagu County, Tasmania.

PREHNITE

This species, which is named after its discoverer, Colonel Prehn, is
found in nodular, yellow and oil-green stones, of which the latter
have very occasionally been cut. It is a little soft, the hardness
being only 6 on Mohs’s scale. The double refraction is large in amount,
0·033, biaxial in character, and positive in sign, the least and the
greatest of the refractive indices being 1·616 and 1·649 respectively;
the specific gravity varies from 2·81 to 2·95. In composition prehnite
is a silicate of aluminium and calcium corresponding to the formula
H_{2}Ca_{2}Al_{2}(SiO_{4})_{3}.

The best material has been found at St. Cristophe, Bourg d’Oisans,
Dauphiné, France.

APATITE

This interesting mineral is found occasionally in attractive green,
blue, or violet stones, but is unfortunately too soft for extensive use
in jewellery, the hardness being only 5 on Mohs’s scale. In composition
it is a fluo-chloro-phosphate of calcium, corresponding to the formula
Ca_{4}[Ca(F,Cl)](PO_{4})_{3}. When pure, it is devoid of colour,
the tints being due to the presence of small amounts of tinctorial
agents. The double refraction is uniaxial in character and negative in
sign, the ordinary index being 1·642 and the extraordinary 1·646; the
specific gravity varies from 3·17 to 3·23. The dichroism is usually
feeble, but sometimes is strong; for instance, in the stones from the
Burma ruby mines (yellow, blue-green). A cut stone might be mistaken
for tourmaline, but is distinguished by its softness, or, when tested
on the refractometer, by its inferior double refraction. It received
its name from ἀπατάειν, deceive, because it was wrongly assigned to at
least half a dozen different species in early days. Moroxite is a name
sometimes given to blue-green apatite.

Beautiful violet stones are found at Ehrenfriedersdorf, Saxony;
Schlaggenwald, Bohemia; and Mount Apatite, Auburn, Androscoggin County,
Maine, U.S.A.; and blue stones come from Ceylon.

DIOPTASE

Though of a pretty, emerald-green colour, dioptase has never been
found in large enough crystals for gem purposes, and it is, moreover,
rather soft, the hardness being only 5 on Mohs’s scale, and has an
easy cleavage. In composition it is a hydrous silicate of copper
corresponding to the formula CuH_{2}SiO_{4}. The double refraction,
which is large in amount, is uniaxial in character, and positive in
sign, the ordinary refractive index being 1·667 and the extraordinary
1·723. Its colour and softness distinguish it from peridot or diopside,
which have about the same refractivity. The name was assigned to the
species by Haüy, from διὰ, through, and ὄπτομαι, see, because the
cleavage directions were distinguishable by looking through the stone.

Dioptase has been found near Altyn-Tübe in the Kirghese Steppes, at
Rezbánya in Hungary, and Copiapo in Chili, and at the mine Mindouli,
near Comba, in the French Congo.

CHAPTER XXXVII

CASSITERITE, ANATASE, PYRITES, HEMATITE

CASSITERITE

Though usually opaque, this oxide of tin, corresponding to the formula
SnO_{2}, has occasionally, but very rarely, been found in small,
transparent, yellow and reddish stones suitable for cutting. The lustre
is adamantine. The refraction is uniaxial in character and positive
in sign, the ordinary index being 1·997 and extraordinary 2·093. The
specific gravity is high, ranging from 6·8 to 7·1. The hardness is on
the whole less than that of quartz, being about 6 to 7 on Mohs’s scale.

ANATASE

This mineral, which is one of the three crystallized forms of titanium
oxide, TiO_{2}, occurs often in small, brown, transparent stones which
occasionally find their way into the market. The lustre is adamantine.
The refraction is uniaxial in character and negative in sign, the
extraordinary index being 2·493 and ordinary 2·554. The specific
gravity varies from 3·82 to 3·95, and the hardness is about 5½ to 6 on
Mohs’s scale.

PYRITES, HEMATITE

These two metallic minerals were employed in ancient jewellery. The
former, sulphide of iron, FeS_{2}, is brass-yellow in colour, and has
a specific gravity 5·2, and hardness 6½ on Mohs’s scale. It is found,
when fresh, in brilliant cubes. The latter, oxide of iron, Fe_{2}O_{3},
has a black metallic lustre, but, when powdered, is red in colour—a
mode of distinguishing it from other minerals of similar appearance.
Its specific gravity is 5·3, and hardness 6½ on Mohs’s scale. In modern
times it has been cut in spherical form to imitate black pearls, but
can easily be recognized by its greater density and hardness. Hematite
is used for signet stones, often with an intaglio engraving.

CHAPTER XXXVIII

OBSIDIAN, MOLDAVITE

Two forms of natural glass have been employed for ornamental purposes.
Obsidian results from the solidification without crystallization of
lava, and corresponds in composition to a granite. The structure
is seldom clear and transparent, and usually contains inclusions
or streaks. The colour is in the mass jet-black, but smoky in thin
fragments, and occasionally greenish. Its property of breaking with a
keen cutting edge, in the same way as ordinary glass, rendered it of
extreme utility to primitive man, who was ignorant of the artificial
substance. The refraction is, of course, single, and the refractive
index approximates to 1·50. The specific gravity varies from 2·3 to
2·5. The hardness is 5 on Mohs’s scale, the same as ordinary glass.

Obsidian is obtained wherever there has been volcanic activity. Vast
mines of great antiquity exist in the State of Hidalgo, Mexico.

Moldavite, which differs in no respect from ordinary green
bottle-glass, is of interest on account of its problematical origin.
Its occurrence in various parts of Bohemia and Moravia cannot be
explained as the result of volcanic agency. It may possibly be the
product of old and forgotten glass factories which at one time existed
on the site. Even meteorites have been suggested as the source. The
physical characters are the same as those of ordinary glass: refraction
single, index 1·51; specific gravity 2·50 and hardness 5½ on Mohs’s
scale. Moldavite also passes under the names of bottle-stone, or
water-chrysolite. A natural glass of the same character has been found
in water-worn fragments in Ceylon, and has been sold as peridot, which
it resembles in colour, but is readily distinguished from it by its
very different physical properties.

PART II—SECTION C

ORNAMENTAL STONES

CHAPTER XXXIX

FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, RHODONITE, AZURITE,
MALACHITE, THULITE, MARBLE, APOPHYLLITE, CHRYSOCOLLA,
STEATITE OR SOAPSTONE, MEERSCHAUM, SERPENTINE

Space will not permit of more than a few words concerning the more
prominent of the numerous mineral species which are employed for
ornamental purposes in articles of virtu or in architecture, but which
for various reasons cannot take rank as gem-stones.

Fluor, a beautiful mineral which is found in its greatest perfection in
England, has enjoyed well-deserved popularity when worked into vases
or other articles. The finest material, deep purple in colour, known
as ‘Blue John,’ came from Derbyshire, but the supply is now exhausted.
The crystallized examples, from Durham, Devonshire, and Cornwall, form
some of the most attractive of museum specimens. The crystals take the
shape of cubes, often twinned, and have an easy octahedral cleavage.
The refraction is single, the index being 1·433. Fluor is noted for its
property of appearing of differing colour by reflected and transmitted
light, and the phenomenon is in consequence known as fluorescence. The
specific gravity is 3·18, and the hardness 4 on Mohs’s scale. Owing to
its low refraction and softness, fluor is not suitable for jewellery.
Clear colourless material is in demand for particular lenses of
microscope objectives.

The lovely blue stone known as lapis lazuli has since the earliest
times been applied to all kinds of decorative purposes, for mosaic
and inlaid work and as the material for vases, boxes, and so on, and
was the original sapphire of the ancients. When ground to powder it
furnishes a fine blue paint, but it has now been entirely superseded
for this purpose by an artificial product. Although to the eye so
homogeneous and uniform in structure, lapis lazuli has been shown
by microscopic examination to be composed of calcite coloured by
three blue minerals in varying proportions. All three belong to the
cubic class of symmetry, and are mainly soda aluminium silicates
in composition; their hardness varies from 5 to 6 on Mohs’s scale.
Lazurite, Na_{4}(NaS_{3}.Al)Al_{2}Si_{3}O_{12}, has specific gravity
varying from 2·38 to 2·45, and hardness about 5 to 5½; haüynite,
(Na_{2},Ca)_{2}(NaSO_{4},Al)Al_{2}Si_{3}O_{12}, is about the same in
specific gravity, 2·4 to 2·5, but slightly harder, 5½ to 6; while
sodalite, Na_{4}(AlCl)Al_{2}Si_{3}O_{12}, is the lightest in density,
2·14 to 2·30, with hardness 5½ to 6, and has a refractive index 1·483.

By far the oldest mines are in the Badakshan district of Afghanistan,
a few miles above Firgamu in the valley of the Kokcha, a branch of
the Oxus, where ruby and spinel are found. It is also found at the
southern end of Lake Baikal, Siberia, and in the Chilian Andes.

Sodalite occurs in beautiful blue masses at Dungannon, Hastings County,
Ontario, Canada, and at Litchfield, Maine, U.S.A. They make excellent
polished stones.

Violane, a massive, dark violet-blue diopside from San Marcel,
Piedmont, Italy, also makes a handsome polished stone.

Rhodonite, silicate of manganese, MnSiO_{3}, possesses a fine red
colour, and makes an attractive stone when cut and polished. It has
very slight biaxial double refraction, the refractivity being about
1·73; the specific gravity is 3·6, and hardness 6. It is found in large
masses near Ekaterinburg in the Ural Mountains, and is quarried as an
ornamental stone.

Both the copper carbonates, azurite or chessylite, and malachite, make
effective polished stones. The latter is also worked into various
ornamental objects; it occurs in fibrous masses, the grained character
of which look well in the polished section. Its colour is a bright
green, to which it owes its name, from μαλακή, mallows. Its composition
is represented by the formula CuCO_{3}.Cu(OH)_{2}, and it is the more
stable form, since azurite is frequently found altered to it. It
has biaxial double refraction, and the indices are about 1·88; the
specific gravity is 4·01, and hardness about 3½ to 4 on Mohs’s scale.
It is found in large masses at the copper mines of Nizhni Tagilsk in
the Ural Mountains, where it is mined as an ornamental stone; it also
accompanies the copper ores in many parts of the world, for instance
Cuba, Chili, and Australia. Azurite, so called on account of its
beautiful blue colour, is rarer, but, unlike malachite, is generally in
the form of crystals. Beautiful specimens have come from Chessy, near
Lyons, France, and Bisbee, Arizona, U.S.A. The composition corresponds
to the formula 2CuCO_{3}, Cu(OH)_{2}. The specific gravity is 3·80, and
hardness about 3½ to 4.

Chrysocolla occurs in blue and bluish-green earthy masses, with
an enamel-like texture, which in some instances can be worked and
polished. Being the result of the decomposition of copper ores, it
varies considerably in hardness, ranging from 2 to 4 on Mohs’s scale.
Its composition approaches to the formula CuSiO_{3}.2H_{2}O, but it
invariably contains impurities. It is very light, the density being
only about 2·2.

Steatite, or soapstone, is a massive foliated silicate of magnesium
corresponding to the formula H_{2}Mg_{3}Si_{4}O_{12}, which is one of
the softest of mineral substances, representing the degree 1 on Mohs’s
scale, but in massive pieces is harder owing to the intermixture of
other substances with it. It has a peculiar greasy feeling to the
touch, due to its softness. The specific gravity is about 2·75. The
Chinese carve images out of the yellowish and brownish pieces.

Meerschaum, a silicate of magnesium corresponding to the formula
H_{4}Mg_{2}Si_{3}O_{10}, is familiar to every smoker as a material for
pipe-bowls. It is very light, the specific gravity being only 2·0, and
soft, the hardness being about 2 to 2½ on Mohs’s scale. When found,
it is pure white in colour, and answers to its name, a German word
signifying sea-foam. It comes from Asia Minor.

Serpentine has been largely used for decorative purposes, as well
as for cameos and intaglios, and formed most of the famous ‘verde
antique.’ Being the result of the decomposition of other silicates
it varies enormously in appearance and characters, but the most
attractive stones are a rich oil-green in colour and resemble jade. The
composition approximates to the formula H_{4}Mg_{3}Si_{2}O_{9}, but it
invariably contains other elements. The hardness varies from 2½ to 4
on Mohs’s scale, according to the minerals contained in the stone; the
specific gravity is about 2·60 and the refractivity 1·570.

The beautiful rose-red stone, thulite, makes a handsome decorative
stone. It has nearly the same composition as epidote (p. 275), and like
it has strong dichroism, the principal colours being yellow, light
rose, and deep rose. The colour is due to manganese. Its refractive
index is about 1·70, specific gravity 3·12, and hardness 6 to 6½ on
Mohs’s scale; it possesses an easy cleavage. Fine specimens come from
Telemark, Norway, and it is therefore called after the old name for
Norway, Thule.

Marble is a massive calcite, carbonate of lime, with the formula
CaCO_{3}. When pure it is white, but it is usually streaked with other
substances which impart a pleasing variety to its appearance. It is
always readily recognized by the immediate effervescence set up when
touched with a drop of acid. Calcite is highly doubly refractive (cf.
p. 40), the extraordinary index being 1·486, and ordinary 1·658, a
difference of 0·172; the specific gravity is 2·71, and hardness 3 on
Mohs’s scale. Lumachelle, or fire-marble, is a limestone containing
shells from which a brilliant, fire-like chatoyancy is emitted
when light is reflected at the proper angle. It sometimes resembles
opal-matrix, but is easily distinguished by its lower hardness and by
its effervescent action with acid. Choice specimens come from Bleiberg
in Carinthia, and from Astrakhan.

Apophyllite has not many characters to commend it, being at the best
faintly pinkish in colour, and always imperfectly transparent. It is
a hydrous silicate of potassium and calcium with the complex formula
(H,K)_{2}Ca(SiO_{3})_{2}.H_{2}O. Its refractivity is about 1·535,
specific gravity 2·5, and hardness 4½ on Mohs’s scale; it possesses
an easy cleavage. It occurs in the form of tetragonal crystals at
Andreasberg in the Harz Mountains, and in the Syhadree Mountains,
Bombay, India.

PART II—SECTION D

ORGANIC PRODUCTS

CHAPTER XL

PEARL, CORAL, AMBER

Although none of the substances considered in this chapter come within
the strict definition of a stone, since they are directly the result of
living agency, yet pearl at least cannot be denied the title of a gem.
Both pearl and coral contain calcium carbonate in one or other of its
crystallized forms, and both are gathered from the sea; but otherwise
they have nothing in common. Amber is of vegetable origin, and is a
very different substance.

PEARL

From that unrecorded day when some scantily clothed savage seeking for
succulent food opened an oyster and found to his astonishment within
its shell a delicate silvery pellet that shimmered in the light of a
tropical sun, down to the present day, without intermission, pearl has
held a place all its own in the rank of jewels. Though it be lacking in
durability, its beauty cannot be disputed, and large examples, perfect
in form and lustre, are sufficiently rare to tax the deepest purse.

The substance composing the pearl is identical with the iridescent
lining—mother-o’-pearl or nacre, as it is termed—of the shell.
Tortured by the intrusion of some living thing, a boring parasite,
a worm, or a small fish, or of a grain of sand or other inorganic
substance, and without means to free itself, the mollusc perforce
neutralizes the irritant matter by converting it into an object of
beauty that eventually finds its way into some jewellery cabinet. Built
up in a haphazard manner and not confined by the inexorable laws of
intermolecular action, a pearl may assume any and every variety of
shape from the regular to the fantastic. It may be truly spherical,
egg- or pear-shaped—pear-drops or pear-eyes, as they are termed—or it
may be quite irregular—the so-called baroque or barrok pearls. The
first is the most prized, but a well-shaped drop-pearl is in great
demand for pendants or ear-rings. The colour is ordinarily white, or
faintly tinged yellowish or bluish, and somewhat rarely, salmon-pink,
reddish, or blackish grey. Perfect black pearls are valuable, but not
as costly as the finest of the white. Though not transparent, pearl is
to a varying extent translucent, and its characteristic lustre—‘orient’
in the language of jewellery—is due to the same kind of interaction
of light reflected from different layers that has been remarked upon
in the case of opal and certain other stones. The translucency varies
in degree, and some jewellers speak of the ‘water’ of pearls just as
in the case of diamonds. If a pearl be sliced across the middle and
the section be examined under the microscope, it will be seen that
the structure consists of concentric shells and resembles that of an
onion. These shells are alternately composed of calcium carbonate in
its crystallized form, aragonite, and of a horny organic matter known
as conchiolin, and they evidently represent the result of intermittent
growth. Because of their composite character, pearls have a specific
gravity ranging from 2·65 to 2·69-2·84-2·89 in the case of pink
pearls—which is appreciably less than that of aragonite, 2·94: the
hardness is about the same, namely, 3½ to 4 on Mohs’s scale. That the
arrangement of the mineral layers is approximately parallel is evinced
by the distinctness of the shadow-edges shown on examination with the
refractometer. Pearls require very careful handling, both because they
are comparatively soft and therefore apt to be scratched, and because
they are chemically affected by acids, and even by the perspiration
from the skin. Acids attack only the calcium carbonate, not the
organic matter; the well-known story therefore of Cleopatra dissolving
a valuable pearl in vinegar, which is moreover, too weak an acid to
effect the solution quickly, must not be accepted too literally.
Pearls are not cut like stones, and therefore as soon as the precious
bloom has once gone, nothing can be done to revive it. Attempts are
sometimes made in the case of valuable pearls to remove the dull skin
and lay bare another iridescent layer underneath, but the operation is
exceedingly delicate. Even with the best of care pearls must in process
of time perish owing to the decay of the organic constituent. Pearls
that have been discovered in ancient tombs crumbled to dust at a touch,
and those formerly in ancient rings have vanished or only remain as
a brown powder, while the garnets or other stones set with them are
little the worse for the centuries that have passed by.

The largest known pearl was at one time in the famous collection
belonging to the banker, Henry Philip Hope. Cylindrical in form, with
a slight swelling at one end, it measures 50 mm. (2 inches) in length,
and 115 mm. (4½ inches) in circumference about the thicker, and 83
mm. (3¼ inches) about the thinner end, and weighs 454 carats. About
three-quarters of it is white in colour with a fine ‘orient,’ and the
remainder is bronze in tint. It is valued at upwards of £12,000. A
large pearl, 300 carats in weight, is in the imperial crown of the
Emperor of Austria, and another, pear-shaped, is in the possession of
the Shah of Persia. A beautiful white India pearl, a perfect sphere in
shape, and 28 carats in weight, is in the Museum of Zosima in Moscow;
it is known as ‘La Pellegrina.’ The ‘Great Southern Cross,’ which
consists of nine large pearls naturally joined together in the shape of
a cross, was discovered in an oyster fished up in 1886 off the beds of
Western Australia. The collection of jewels in the famous Green Vaults
at Dresden contains a number of pearls of curious shapes.

Large pearls are sold separately, while the small pearls known as
‘seed’ pearls come into the market bored and strung on silk in
‘bunches.’ The unit of weight is the pearl grain, which is a quarter
of a carat, and the rate of price depends on the square of the weight
in grains. The rate per unit or base varies from 6d. to 50s. according
to the shape and quality of the pearl. Spherical pearls command the
best prices, next the pearl-drops, and lastly the buttons; but whatever
the shape, it is imperative that the pearl have ‘orient,’ without which
it is valueless. The cheaper grades of pearls are sold by the carat.

[Illustration: _PLATE XXX_

NATIVES DRILLING PEARLS]

For use in necklaces and pendants pearls are bored with a steel drill,
and threaded with silk, an easy operation on account of their softness.
They harmonize well with diamonds. Small pearls are often set as a
frame to large coloured stones, to which they form an admirable foil.
Pearls set in rings or anywhere where the upper half alone would show
are generally sawn in halves; ‘button’ pearls find an extensive use in
modern rings.

Any mollusc, whether of the bi-valve or the uni-valve type, which
possesses a nacreous shell, has the power of producing pearls,
but only two, the pearl-oyster, _Meleagrina margaritifera_, and
the pearl-mussel, _Unio margarifer_, repay the cost of systematic
fishing. The outside of the shell is formed of the horny matter called
conchiolin; while the inside is composed of two coats, of which
the outer consists of alternate layers of conchiolin and calcium
carbonate in its crystallized form, calcite, and the inner of the same
organic matter, but with calcium carbonate in its other crystallized
form, aragonite. The latter coat forms the nacreous lining known as
mother-o’-pearl, which is identical in consistency with pearl, but
somewhat more transparent. The iridescence of mother-o’-pearl is due
not only to the fact that it is composed of a succession of thin
translucent layers, but also to the fact that these layers overlap
like slates on a house, and form a series of fine parallel lines on
the surface; diffraction therefore as well as interference of light
takes place, and a similar diffraction phenomenon is displayed even
by a cast of the inside of the shell. The animal has the property of
secreting calcium carbonate, which it absorbs from the sea-water, in
both its crystallized conditions as well as conchiolin. At the outer
rim it secretes conchiolin, further in calcite, and at the very inside
aragonite. The shape and appearance of a pearl therefore depend on
the position in which the intruding substance is situated within the
shell. The most perfect pearl has been in intermittent motion in the
interior of the mollusc, and has received successive coats according
to the position in which it happened to be. A parasite that bores
into the shell is walled up at the point of entrance, and a wart- or
blister-pearl results. The thinner the successive coats the finer
the lustre. Pearls have even been discovered embedded in the animal
itself. The number of pearls found in a shell depends on the number of
times the living host was compelled to seal up some irritant object,
and may vary from one up to the eighty-seven which are said to have
been found in an Indian oyster. That an oyster thus distinguished has
not led a happy existence is testified by the distorted shape of its
shell, a clue that guides the pearl-fishers in their search. Moreover,
pearl-oysters never have thick nacreous shells, and on the other hand
molluscs with fine mother-o’-pearl seldom contain pearls.

Beautiful white and silvery pearls are found in a small oyster that
lives at a depth of 6 to 13 fathoms (11-24 m.) in the Gulf of Manaar,
off the coast of Ceylon. About seven-eighths, however, of the pearls
that come into the market are obtained from a larger oyster which
has its home on the Arabian coast of the Persian Gulf. These famous
fisheries have been known since very early times. The pearls found
here are more yellowish than those from Ceylon, but are nevertheless
of excellent quality. The pearl fisheries off the north-west coast of
Western Australia and off Venezuela are also not unimportant, and fine
black pearls have been supplied by molluscs from the Gulf of Mexico.

[Illustration: _PLATE XXXI_

METAL FIGURES OF BUDDHA INSERTED IN A PEARL-OYSTER]

[Illustration: _PLATE XXXII_

FIG. 1

FIG. 2

SECTIONS OF CULTURE PEARL

FIG. 1. IN THE OYSTER. FIG. 2. WHEN FINISHED.

A. PEARLY DEPOSIT. B. PIECE OF MOTHER-O’-PEARL INSERTED IN THE OYSTER.
C. OUTER SHELL OF THE OYSTER. D. MOTHER-O’-PEARL BACK ADDED.]

The Chinese have long made a practice of introducing into the shell of
a pearl-oyster little tin images of Buddha in order that they may be
coated with the nacreous secretion. The Japanese have during recent
years made quite an industry of stimulating the efforts of the mollusc
by cementing small pieces of mother-o’-pearl to the interior surface
of the shell (Plate XXXII, Fig. 1); these ‘culture’ pearls, as they
are termed, are recognizable by examination of the back. About a year
has to elapse before a coating of a tenth of a millimetre is formed,
and another two years must pass before the thickness is doubled. After
removal the piece of mother-o’-pearl, which is now coated with several
nacreous layers, is cemented to a piece of ordinary mother-o’-pearl,
and the lower portion is ground to the usual symmetrical shape (Plate
XXXII, Fig. 2). Blister pearls are often similarly treated. In both
cases, however, the ‘orient’ is deficient in quality.

The finest mother-o’-pearl is supplied by a mollusc found in the sea
near the islands lying between Borneo and the Philippines, and fine
material is found at Shark Bay and off Thursday Island.

CORAL

Coral ranks far below pearl and meets with but limited appreciation.
It is common enough in warm seas, but the only kind which finds its
way into jewellery is the rose or red-coloured coral—the noble coral,
_Corallium nobile_ or _rubrum_. It consists of the axial skeleton of
the coral polyp, and is built up of hollow tubes fitting one within
the other. The composition is mainly calcium carbonate with a little
magnesium carbonate and a small amount of organic matter. The former
of the mineral substances is in the form of calcite, and the crystals
are arranged in fibrous form radiating at right angles to the axis of
the coral. The specific gravity varies from 2·6 to 2·7, being slightly
under that of calcite, and the hardness is somewhat greater, being
about 3¾ on Mohs’s scale.

The best red coral is found in the Mediterranean Sea off Algiers and
Tunis in Africa, and Sicily and the Calabrian Coast of Italy. The
industry of shaping and fashioning the coral is carried on almost
entirely in Italy. Coral is usually cut into beads, either round or
egg-shaped, and used for necklaces, rosaries, and bracelets. The best
quality fetches from 20s. to 30s. per carat.

AMBER

This fossil resin, yellow and brownish-yellow in tint, finds an
extensive use as the material for mouthpieces of pipes, cigar and
cigarette-holders, umbrella-handles, and so on, and is even locally
cut for jewellery, although its extreme softness, its hardness being
only 2½ on Mohs’s scale, quite unfits it for such a purpose. It is
only slightly denser than water, the specific gravity being about
1·10. Since the structure is amorphous the refraction is single, the
index being about 1·540. Amber, being a very bad conductor of heat, is
perceptibly warm to the touch. Its property of becoming electrified by
friction attracted early attention, and from the Greek name for it,
ἤλεκτρον, is derived our word electricity.

Amber is washed up by the sea off the coasts of Sicily and Prussia,
and of Norfolk and Suffolk in England. The finest examples, which are
picked up off the shore of Catania in Sicily, are distinguished by a
fine bluish fluorescence, resembling that seen in lubricating oil; such
pieces command good prices.

A recent resin, pale yellow in colour, known as kauri-gum, is found in
New Zealand, where it is highly valued.

TABLES

TABLE I

_Chemical Composition of Gem-Stones_

(_a_) ELEMENTS—

Diamond C

(_b_) OXIDES—

Corundum Al_{2}O_{3}
Quartz SiO_{2}
Chalcedony SiO_{2}
Opal SiO_{2}.nH_{2}O

(_c_) ALUMINATES—

Spinel MgAl_{2}O_{4}
Chrysoberyl BeAl_{2}O_{4}

(_d_) SILICATES—

Phenakite Be_{2}SiO_{4}
Dioptase H_{2}CuSiO_{4}
Peridot Mg_{2}SiO_{4}
Zircon ZrSiO_{4}
Enstatite MgSiO_{3}
Diopside CaMg(SiO_{3})_{2}
Nephrite CaMg_{3}(SiO_{3})_{4}
Sphene CaTiSiO_{5}
Benitoite BaTiSi_{3}O_{9}
Andalusite Al(AlO)SiO_{4}
Kyanite (AlO)_{2}SiO_{3}
Topaz [Al(F,OH)]_{2}SiO_{4}
Epidote Ca_{2}(Al,Fe)_{2}(AlOH)(SiO_{4})_{3}
Euclase Be(AlOH)SiO_{4}
Prehnite H_{2}Ca_{2}Al_{2}(SiO_{4})_{3}
Iolite H_{2}(Mg,Fe)_{4}Al_{8}Si_{10}O_{37}
{ Hessonite Ca_{3}Al_{2}(SiO_{4})_{3}
_Garnet_ { Pyrope Mg_{3}Al_{2}(SiO_{4})_{3}
{ Almandine Fe_{3}Al_{2}(SiO_{4})_{3}
{ Andradite Ca_{3}Fe_{2}(SiO_{4})_{3}
Beryl Be_{3}Al_{2}(SiO_{3})_{6}
Spodumene LiAl(SiO_{3})_{2}
Jadeite NaAl(SiO_{3})_{2}
Moonstone KAlSi_{3}O_{8}
Tourmaline{12SiO_{2}.3B_{2}O_{3}.(9-x)[(Al,Fe)_{2}O_{3}].3x[
{(Fe,Mn,Ca,Mg,K_{2},Na_{2},Li_{2},H_{2})O].3H_{2}O
Axinite HCa_{3}Al_{2}B(SiO_{4})_{4}
Idocrase (Ca,Mn,Mg,Fe)_{2}(Al,Fe)(OH,F)]Si_{2}O_{7}

(_e_) PHOSPHATES—

Beryllonite NaBePO_{4}
Apatite Ca_{5}(F,Cl)(PO_{4})_{3}
Turquoise CuOH.6[Al(OH)_{2}].H_{5}.(PO_{4})_{4}

TABLE II

_Colour of Gem-Stones_

_Colourless and White._—Diamond, corundum (white sapphire), topaz,
quartz (rock-crystal), zircon (when ‘fired’), moonstone; rarely
beryl, tourmaline; among the less common species, phenakite,
spodumene (colourless kunzite), beryllonite.

_Yellow._—Diamond, topaz, corundum (yellow sapphire), quartz
(citrine, Scotch or occidental topaz), tourmaline, zircon,
sphene, spodumene, beryl.

_Pink and Lilac._—Corundum (pink sapphire), spinel (balas-ruby),
tourmaline (rubellite), topaz (usually when ‘fired’), spodumene
(kunzite), beryl (morganite), quartz (rose-quartz).

_Red._—Corundum (ruby), garnet (pyrope, almandine), spinel
(balas-ruby), tourmaline (rubellite), zircon, opal (fire-opal).

_Green._—Beryl (emerald, aquamarine), peridot, corundum,
tourmaline, chrysoberyl (including alexandrite), zircon, garnet
(demantoid); among less common species, spodumene (hiddenite),
euclase, diopside, idocrase, epidote, apatite, obsidian; rarely
diamond; also semi-opaque, turquoise, jade.

_Blue._—Corundum (sapphire), spinel, topaz, tourmaline, zircon;
among the less common species, kyanite, iolite, benitoite,
apatite; rarely diamond; also semi-opaque, turquoise, lapis
lazuli, sodalite.

_Violet and Purple._—Quartz (amethyst), corundum (oriental
amethyst), spinel (almandine-spinel), garnet (almandine),
spodumene (kunzite), apatite.

_Brown._—Diamond, tourmaline, quartz (smoky-quartz); among the less
common species, andalusite, axinite, sphene.

TABLE III

_Refractive Indices of Gem-Stones_[8]

Opal 1·454
Moonstone 1·53 1·54
Iolite 1·543 1·551
Quartz 1·544 1·553
Beryllonite 1·553 1·565
Beryl 1·578 1·585
Turquoise 1·61 1·65
Topaz 1·618 1·627
Andalusite 1·632 1·643
Tourmaline 1·626 1·651
Apatite 1·642 1·646
Phenakite 1·652 1·667
Euclase 1·651 1·670
Spodumene 1·660 1·675
Enstatite 1·665 1·674
Peridot 1·659 1·697
Axinite 1·674 1·684
Diopside 1·685 1·705
Idocrase 1·714 1·719
Spinel 1·726
Kyanite 1·72 1·73
Epidote 1·735 1·766
Garnet (Hessonite) 1·745
Chrysoberyl 1·746 1·753
Garnet (Pyrope) 1·755
Benitoite 1·757 1·804
Corundum 1·761 1·770
Garnet (Almandine) 1·790
Zircon (a) 1·815
Garnet (Demantoid) 1·885
Sphene 1·901 1·985
Zircon (b) 1·927 1·980
Diamond 2·417

TABLE IV

_Colour-Dispersion of Gem-Stones_[9]

Moonstone ·012
Quartz ·013
Beryl ·014
Topaz ·014
Chrysoberyl ·015
Tourmaline ·017
Spodumene ·017
Corundum ·018
Peridot ·020
Spinel ·020
Garnet (Almandine) ·024
Garnet (Pyrope) ·027
Garnet (Hessonite) ·028
Zircon ·038
Diamond ·044
Sphene ·051
Garnet (Demantoid) ·057

TABLE V

_Character of the Refraction of Gem-Stones_

(_a_) SINGLE—

Diamond, spinel, garnet, opal.
Diamond and garnet frequently display local double refraction.

(_b_) UNIAXIAL, POSITIVE—

Quartz ·009
Phenakite ·015
Benitoite ·047
Zircon (b) ·053
Quartz exhibits circular polarization.

(_c_) UNIAXIAL, NEGATIVE—

Apatite ·004
Idocrase ·005
Beryl ·007
Corundum ·009
Tourmaline ·025

(_d_) BIAXIAL, POSITIVE—

Chrysoberyl ·007
Topaz ·009
Enstatite ·009
Spodumene ·015
Euclase ·019
Diopside ·020
Peridot ·038
Sphene ·084

(_e_) BIAXIAL, NEGATIVE—

Moonstone ·006
Iolite ·008
Axinite ·010
Andalusite ·011
Beryllonite ·012
Kyanite ·016
Epidote ·031

TABLE VI

_Dichroism of Gem-Stones_

(_a_) STRONG

Corundum, tourmaline, alexandrite, spodumene, andalusite, iolite,
epidote, axinite.

(_b_) DISTINCT

Emerald, topaz, quartz, peridot, chrysoberyl, enstatite, euclase,
idocrase, kyanite, sphene, apatite.

(_c_) WEAK

Beryl, diopside.

TABLE VII

_Specific Gravities of Gem-Stones_

Opal 2·15
Moonstone 2·57
Iolite 2·63
Quartz 2·66
Beryl 2·74
Turquoise 2·82
Beryllonite 2·84
Phenakite 2·99
Euclase 3·07
Tourmaline 3·10
Enstatite 3·10
Andalusite 3·18
Spodumene 3·18
Apatite 3·20
Axinite 3·28
Diopside 3·29
Epidote 3·37
Peridot 3·40
Idocrase 3·40
Sphene 3·40
Diamond 3·52
Topaz 3·53
Spinel 3·60
Kyanite 3·61
Garnet (Hessonite) 3·61
Benitoite 3·64
Chrysoberyl 3·73
Garnet (Pyrope) 3·78
Garnet (Demantoid) 3·84
Corundum 4·03
Garnet (Almandine) 4·05
Zircon (a) 4·20
Zircon (b) 4·69

TABLE VIII

_Degrees of Hardness of Gem-Stones_

5. Kyanite (5-7), apatite, lapis lazuli
5½. Enstatite, beryllonite, sphene
6. Opal, moonstone, turquoise, diopside
6½. Spodumene, peridot, garnet (demantoid), benitoite, idocrase,
epidote, axinite, jade (nephrite)
7. Iolite, quartz, tourmaline, jade (jadeite)
7¼. Garnet (hessonite, pyrope)
7½. Beryl, garnet (almandine), zircon, phenakite, euclase, andalusite
8. Topaz, spinel
8½. Chrysoberyl
9. Corundum
10. Diamond

TABLE IX.—DATA

_Densities of Water and Toluol at Ordinary Temperatures_

+-----------------------------+----------+----------+
| TEMPERATURE | WATER | TOLUOL |
+-----------------------------+----------+----------+
| Centigrade | Fahrenheit | | |
| | | | |
| 14° | 57·2° | 0·9994 | 0·8697 |
| 15° | 59·0° | 0·9992 | 0·8687 |
| 16° | 60·8° | 0·9990 | 0·8677 |
| 17° | 62·6° | 0·9988 | 0·8667 |
| 18° | 64·4° | 0·9986 | 0·8657 |
| 19° | 66·2° | 0·9985 | 0·8647 |
| 20° | 68·0° | 0·9983 | 0·8637 |
| 21° | 69·0° | 0·9981 | 0·8627 |
| 22° | 71·6° | 0·9979 | 0·8617 |
| 23° | 73·4° | 0·9977 | 0·8607 |
+-----------------------------+----------+----------+

1 English carat = 0·2053 gram
1 Metric carat = 0·2000 (one-fifth) gram
1 oz. Av. = 28·35 grams
1 lb. Av. = 0·4536 kilogram
1 inch = 25·4 millimetres
1 foot = 0·3048 metre
1 yard = 0·9144 metre
1 mile = 1·6093 kilometre

INDEX

Absorption, 53, 59

Absorption spectra, 59

Achroite, 220, 221

Adularia, 255

Agate, 247

Akbar Shah diamond, 163

Alalite, 272

Albite, 254

Alexandrite, 54, 60, 233
Scientific, 122

Almandine, 60, 214
Oriental, 112, 172
spinel, 112, 204

Amazon-stone, 255

Amber, 83, 298

Amethyst, 239, 242
Oriental, 111, 172, 239

Anatase, 281

Andalusite, 274

Andradite, 216

Anomalous refraction, 47

Anorthite, 254

Apatite, 279

Apophyllite, 290

Aquamarine, 184, 193

Arizona-ruby, 213

Artificial stones, 124

Asteria, 38, 177

Asterism, 38

Australia stones, 154, 174, 182, 195, 213, 216, 227, 232, 252,
288

Austrian Yellow diamond, 165

Aventurine, 240, 241

Axes, Crystallographic, 9
Optic, 49

Axinite, 278

Azure-quartz, 244

Azurite, 287

Balas-ruby, 203

Barnato, Barnett, 145

Baroque, Barrok, pearls, 292

Bastite, 272

Benitoite, 267

Berquem, Louis de, 90, 161

Beryl, 184

Beryllonite, 270

Bezel facet, 92

Biaxial double refraction, 45, 49, 57

Bisectrix, 45, 49

Black diamond, 129

Black lead, 129

Black opal, 249, 250

Black Prince’s ruby, 206

Blister-pearl, 296

Bloodstone, 247

Blue felspar, 255

Blue ground, 143, 147

Blue John, 285

Boart, 103, 129, 133

Bohemian garnet (pyrope), 207, 212

Bone turquoise, 259

Boodt, A. B. de, 132, 213

Borgis, Hortensio, 161

Borneo stones, 154, 170

Bort, _v._ Boart, 103, 129, 133

Bottle-stone, 284

Boule, 118

Bowenite, 263

Braganza diamond, 170

Brazil stones, 138, 165, 166, 169, 194 _et seq._, 201, 215, 223,
236, 243, 244, 248, 266, 269, 270, 274

Brazilian emerald, 111, 220, 221
peridot, 221
sapphire, 111, 221
topaz, 111, 197

Brilliant form of cutting, 92

Brilliant, Scientific, 122

Bristol diamonds, 243

Bruting, 100

Burma stones, 178, 205, 223, 227, 263

Button-pearl, 295

Byes, Bywaters, 136, 150

Cabochon form of cutting, 88

Cacholong, 251

Cairngorm, 239

Callaica, callaina, callais, 258

Calcite, 40, 289

California stones, 156, 195, 202, 224, 259, 265, 267, 275

Californite, 264, 275

Cape-ruby, 213

Carat weight, 72, 84

Carbon, 129

Carbonado, 129

Carborundum, 105

Carbuncle, 89, 215

Carnelian, 247

Cascalho, 139

Cassiterite, 281

Cat’s-eye (chrysoberyl), 38, 90, 233
(quartz), 39, 90, 240
(tourmaline), 39, 219
Hungarian, 244

Ceylon stones, 181, 195, 201, 205, 212, 215, 216, 223, 232, 236,
237, 243, 244, 255, 267, 274, 279, 284

Ceylonese peridot (tourmaline), 221

Ceylonite, 204

Chalcedony, 246

Chatoyancy, 38

Chert, 247

Chessylite, 287

Chrysoberyl, 233

Chrysocolla, 288

Chrysolite (chrysoberyl), 233
(peridot), 225

Chrysoprase, 247

Church, Sir Arthur, 61, 211, 231

Cinnamon-stone, 211

Citrine, 239

Cleavage, 80, 100, 149

Close goods, 149

Colenso diamond, 131

Colour, 53

Colour dispersion, 20, 97

Conchiolin, 293

Coral, 298

Cordierite, 266

Cornish diamonds, 243

Corundum, 172

Crocidolite, 39, 240

Crookes, Sir William, 132, 153

Cross facet, 93

Crystal, 6, 7, 8
Rock-, 97

Cubic system, 8

Culet facet, 93

Cullinan diamond, 94, 100, 168

Culture pearls, 297

Cumberland diamond, 164

Cyanite (Kyanite), 79, 273

Cymophane, 234

Darya-i-nor diamond, 162

De Beers diamonds, 167

Demantoid, 216

Density, 63

Deviation, Minimum, 30

Diamond, Characters of, 128
cutting, 90
gauges, 86
Glaziers’, 135
mining, 146
Occurrence of, in—
Borneo, 154
Brazil, 139
German South-West Africa, 155
India, 138
New South Wales, 154
Rhodesia, 155
South Africa, 139
Origin of, 151
-point, 91
-rose, 92
-table, 91

Diamonds, Classification of, 136, 149
Historical, 157
Prices of, 135

Dichroism, 55

Dichroite, 266

Dichroscope, 55

Diffusion column, 65

Diopside, 272

Dioptase, 280

Dispersion, Colour, 20, 24, 97

Disthene, 273

Dop, 102

Double refraction, 28, 40

Doublet, 125

Dresden diamond, 171

Drop-stone, 94

Duke of Devonshire’s emerald, 191

Edwardes ruby, 175

Electrical characters, 82

Emerald, 89, 184
Brazilian, 220, 221
Evening, 225
Oriental, 111, 172
Scientific, 122
Uralian, 216

Emeraldine, 247

Emery, 175

English Dresden diamond, 166

Enstatite, 271

Epidote, 275

Essence d’Orient, 126

Essonite (Hessonite), 211

Euclase, 269

Eugénie diamond, 164

Evening emerald, 225

Excelsior diamond, 167

Extinction, 45

Faceting machine, 105

False topaz, 239

Felspar, 254

Fire, 20, 96

Fire-marble, 289

Fire-opal, 251

Flats, 150

Flêches d’amour, 240

Flint, 247

Floors, 147

Fluor, 285

Frémy, E., 115

Garnet, 207
Green, 271

Gaudin, M. A. A., 115

Gauges, Diamond, 86

Girdle, 92

Glass, 7, 124

Gnaga Boh ruby, 180

Goniometer, 30

Grain, Pearl, 86

Graphite, 129

Greaser, 149

Great Mogul diamond, 161

Great Southern Cross group of pearls, 294

Great Table diamond, 162

Great White diamond, 167

Green garnet, 271

Greenstone, 261

Grossular, 211

Habit, 12

Hardness, 78

Haüynite, 286

Heavy liquids, 64

Hematite, 282

Hessonite, 211

Hexagonal system, 10

Hiddenite, 266

Hope cat’s-eye, 237
chrysolite, 237
diamond, 170
pearl, 294
sapphire, 121

Hornstone, 247

Hungarian cat’s-eye, 244

Hyacinth, 211, 228

Hydrophane, 250

Hydrostatic weighing, 72

Hypersthene, 271

Iceland-spar, 40, 44

Idocrase, 274

Imitation stones, 124

Imperial diamond, 167

Index of refraction, 16

India stones, 137, 181, 194, 215, 243, 244, 248, 290

Indicators, 65

Indicolite, 221

Interference of light, 39, 48

Iolite, 266

Iris, 240

Isle of Wight diamonds, 243

Isomorphous replacement, 13, 19

Jacinth, 211, 228

Jade, 260

Jadeite, 262

Jargoon, 228

Jasper, 247

Jehan Ghir Shah diamond, 163

Jigger, 149

Jubilee diamond, 167

Kauri-gum, 299

Khiraj-i-Alam ruby, 206

Kimberlite, 152

King topaz, 181, 201

Klein’s solution, 67

Koh-i-nor diamond, 137, 158

Kunz, Dr. G. F., 186, 224, 262, 265

Kunzite, 265

Kyanite, 79, 273

Labradorite, 255

La Pellegrina pearl, 294

Lapis lazuli, 286

Lazurite, 286

Lozenge facet, 93

Lumachelle, 289

Lustre, 37

Maacles, Macles, 12, 150

Madagascar stones, 195, 224, 243, 265, 266

Malachite, 287

Malacolite, 272

Manufactured stones, 113

Marble, 289

Mattan diamond, 155, 170

Matura diamonds, 232

Mazarin, Cardinal, 92

Meerschaum, 288

Mêlée, 136

Methylene iodide, 26, 66

Metric carat, 85, 87

Milky-quartz, 240

Minimum deviation, 30

Mocha-stone, 247

Moe’s gauge, 87

Mohs’s scale of hardness, 78

Moissan, Henri, 153

Moldavite, 283

Monoclinic system, 11

Moon of the Mountains diamond, 162

Moonstone, 39, 255

Morganite, 186, 195

Moroxite, 279

Moss-agate, 247

Mother-of-emerald, 240

Mother-o’-pearl, 292

Nacre, 292

Napoleon diamond, 164

Nassak diamond, 163

Negative double refraction, 45

Nephrite, 261

Nicol’s prism, 44

Nizam diamond, 162

Obsidian, 283

Occidental topaz, 111, 239

Odontolite, 259

Off-coloured diamonds, 130

Olivine (demantoid), 216
(peridot), 225

Onyx, 247

Opal, 39, 249
Fire, 251
-matrix, 251

Opalescence, 39

Optical anomalies, 47

Optic axes, 49

Oriental almandine, 112, 172
amethyst, 111, 172
emerald, 111, 172
topaz, 111, 172

Orient of pearls, 292

Orloff diamond, 160

Orthoclase, 254

Orthorhombic system, 11

Pacha of Egypt diamond, 165

Paste, 47, 124

Paul I diamond, 171

Pavilion, 93

Pavilion facet, 93

Pear-drop pearls, 292

Pear-eye pearls, 292

Pearl, 291
grain, 86
imitations, 126

Pendeloque, 94

Peridot, 225
Brazilian, 221
Ceylonese, 221

Peruzzi, Vincenzio, 92

Phenakite, 269

Pigott diamond, 164

Pipes, 152

Pistacite, 275

Pitt diamond, 100, 159

Plasma, 247, 264

Pleochroism, 57

Pleonaste, 204

Pliny, 6, 88, 138, 184, 191, 241, 249

Polar Star diamond, 163

Polarization, 42

Porter-Rhodes diamond, 166

Positive double refraction, 45

Prase, 240, 247

Prehnite, 278

Pycnometer, 75

Pyrites, 282

Pyrope, 212

Quartz, 50, 238

Quoin facet, 93

Rainbow-quartz, 240

Reconstructed stones, 116

Reef, 144

Reflection of light, 14

Refraction of light, 15

Refractive index, 16

Refractometer, 22, 50

Regent diamond, 100, 159

Retgers’s salt, 69

Rhodes, Cecil J., 145

Rhodesia stones, 155, 183, 213, 236

Rhodolite, 62, 214

Rhodonite, 287

Rock-crystal, 97, 239

Rock-drill, 134

Röntgen rays, 83

Rose form of cutting, 91

Rose-quartz, 240

Rospoli sapphire, 182

Rotation of plane of polarization, 50

Rubellite, 220, 223

Rubicelle, 203

Ruby, 98, 110, 172
Balas-, 203
Cape-, 213

Sancy diamond, 161

Sapphire, 98, 110, 172
Brazilian (tourmaline), 221
-quartz, 244
Water- (iolite), 266
Water- (topaz), 201

Sard, 247

Sardonyx, 247

Saussurite, 263

Schorl, 221

Scientific alexandrite, 122
brilliant, 122
emerald, 122
topaz, 121

Scotch topaz, 239

Seed pearls, 294

Serpentine, 289

Setting of gem-stones, 107

Shah diamond, 163

Sheen, 39

Shepherd’s Stone diamond, 163

Siam stones, 180

Siberia and Asiatic Russia stones, 182, 188, 194, 201, 217, 223,
236, 244, 256, 262, 269, 270, 287

Siberite, 221

Siderite, 244

Silver-thallium nitrate, 69

Skew facet, 93

Skill facet, 93

Smoky quartz, 240

Snell’s laws, 16

Soapstone, 288

Sodalite, 286, 287

Sonstadt’s solution, 67

South Africa stones, 139 _et seq._, 166, 167 _et seq._, 213,
232, 244, 264, 271

Spanish topaz, 239

Specific gravity, 63

Specific-gravity bottle, 75

Spectroscope, 59

Spectrum, 20, 25

Spectrum, Absorption, 59

Spessartite, 216

Sphene, 276

Spinel, 203

Spodumene, 265

Spotted stones, 149

Star-facet, 92

Star of Africa diamond, 168

Star of Este diamond, 165

Star of Minas diamond, 169

Star of South Africa diamond, 141, 166

Star of the South diamond, 139, 165

Starstones, 38, 177

Steatite, 288

Step form of cutting, 98

Stewart diamond, 166

Strass, 124

Sunstone, 255

Synthetical stones, 113

Syriam, Syrian, garnet, 215

Table facet, 92

Table form of cutting, 91

Tavernier, J. B., 91, 129, 137, 161, 162, 170

Templet facet, 92

Tetragonal system, 9

Thulite, 289

Tiffany diamond, 171

Tiger’s-eye, 39, 240

Timur ruby, 206

Titanite, 276

Topaz, 197
Brazilian, 197
False, 239
Occidental, 111, 239
Oriental, 111, 173
Scientific, 121
Scotch, 239
Spanish, 239

Topazolite, 216

Total-reflection, 18, 21

Tourmaline, 43, 219

Trap form of cutting, 98

Trichroism, 57

Triclinic system, 12

Triplet, 126

Turquoise, 257

Turquoise-matrix, 258

Tuscany diamond, 165

Twinning, 12, 47

Uniaxial double refraction, 45, 48 57

Uralian emerald, 217

Uvarovite, 218

Variscite, 259

Verdite, 264

Verneuil, A. V. L., 116

Vesuvianite, 274

Victoria diamond, 167

Violane, 287

Wart-pearl, 296

Water (of diamonds), 129
(of pearls), 292

Water-chrysolite, 284
-sapphire (iolite), 266
-sapphire (topaz), 201

White opal, 249

White Saxon diamond, 165

Wollaston, W. H., 133

X-rays, 83

Yellow ground, 143

Zircon, 228

_Printed by_ MORRISON & GIBB LIMITED, _Edinburgh_

+--------------------------------------------------------------------+
| FOOTNOTES: |
| |
| [1] The word medium is employed by physicists to express any |
| substance through which light passes, and includes solids such as |
| glass, liquids such as water, and gases such as air; the nature |
| of the substance is not postulated. |
| |
| [2] Methylene iodide must be heated almost to boiling-point to |
| enable it to absorb sufficient sulphur; but caution must be |
| exercised in the operation to prevent the liquid boiling over |
| and catching fire, the resulting fumes being far from pleasant. |
| It is advisable to verify by actual observation that the liquid |
| is refractive enough not to show any shadow-edge in the field of |
| view of the refractometer. |
| |
| [3] γωνία, angle; μέτρον, measure. For details of the |
| construction, adjustment, and use of this instrument the reader |
| should refer to textbooks of mineralogy or crystallography. |
| |
| [4] A cleavage flake of topaz may conveniently be used to show |
| the phenomenon, but owing to the great width of the angle the |
| “eyes” are invisible. |
| |
| [5] In accordance with the usual custom the angle between the |
| facets is taken as that between their normals, or the supplement |
| of the salient angle. |
| |
| [6] The word paste is derived from the Italian, _pasta_, food, |
| being suggested by the soft plastic nature of the material used |
| to imitate gems. |
| |
| [7] Cf. below, p. 149. |
| |
| [8] The least and the greatest of the refractive indices of |
| doubly refractive species are given. |
| |
| [9] The dispersion is the difference of the refractive indices |
| corresponding to the B and G lines of the solar spectrum. The |
| value for crown-glass is ·016. |
| |
+--------------------------------------------------------------------+

INTERESTING AND IMPORTANT BOOKS

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Transcriber’s Notes:
- Text enclosed by underscores is in italics (_italics_).
- Redundant title page has been removed.
- Blank pages have been removed.
- Front publication list moved to the back.
- Silently corrected typographical errors.
- Where possible Unicode fractions have been used, otherwise they are
formatted as example “1-5/16”.

*** End of this LibraryBlog Digital Book "Gem-Stones and their Distinctive Characters" ***

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