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Title: The Chemistry, Properties and Tests of Precious Stones
Author: Mastin, John, 1865-
Language: English
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Libraries.)
THE CHEMISTRY,
PROPERTIES AND TESTS OF
PRECIOUS STONES
* * * * *
BY THE SAME AUTHOR
THE STOLEN PLANET. (2nd edition.) 3s. 6d.
THROUGH THE SUN IN AN AIRSHIP. 6s.
THE IMMORTAL LIGHT. (2nd edition.) 6s.
C. GRIFFIN AND CO., LTD.
THE AUTOBIOGRAPHY OF A PICTURE.
(2nd edition.) 3s. 6d.
THIS WORKADAY WORLD. (In the Press.)
HENRY J. DRANE.
PEPPER'S BOY'S PLAYBOOK OF SCIENCE.
(New edition.) Now in Press, revised, re-written and re-illustrated by
DR. JOHN MASTIN.
GEORGE ROUTLEDGE AND SONS, LTD.
ETC. ETC.
* * * * *
THE CHEMISTRY, PROPERTIES
AND TESTS
OF
PRECIOUS STONES.
BY
JOHN MASTIN, M.A. D.SC. PH.D. LITT.D.
F.S A.SCOT. F.L.S. F.C.S. F.R.A.S. F.R.M.S. R.B.A.
_Author of "Parasites of Insects," "The True Analysis of Milk,"
"Plate-Culture and Staining of Amoebæ," etc., etc._
_London_
E. & F. N. SPON, LIMITED, 57 HAYMARKET
_NEW YORK_
SPON & CHAMBERLAIN, 123 LIBERTY STREET
1911
Transcriber's note:
For Text: A word surrounded by a cedilla such as ~this~ signifies that
the word is bolded in the text. A word surrounded by underscores like
_this_ signifies the word is italics in the text. The italic and bold
markup for single italized letters or "foreign" abbreviations are
deleted for easier reading.
For numbers and equations: Parentheses have been added to clarify
fractions. Underscores before bracketed numbers in equations denote a
subscript.
CONTENTS
CHAPTER PAGE
I INTRODUCTORY 1
II THE ORIGIN OF PRECIOUS STONES 7
III PHYSICAL PROPERTIES--(A) CRYSTALLINE STRUCTURE 13
IV " " (B) CLEAVAGE 19
V " " (C) LIGHT 26
VI " " (D) COLOUR 32
VII " " (E) HARDNESS 39
VIII " " (F) SPECIFIC GRAVITY 45
IX " " (G) HEAT 52
X " " (H) MAGNETIC AND ELECTRIC INFLUENCES 57
XI THE CUTTING OF PRECIOUS STONES 62
XII IMITATIONS, AND SOME OF THE TESTS OF PRECIOUS STONES 70
XIII VARIOUS PRECIOUS STONES 80
XIV " " " (_continued_) 88
XV " " " " 98
PREFACE
Some little time ago certain London diamond merchants and wholesale
dealers in precious stones made the suggestion to me to write a work on
this section of mineralogy, as there did not appear to be any giving
exactly the information most needed.
Finding there was a call for such a book I have written the present
volume in order to meet this want, and I trust that this handbook will
prove useful, not only to the expert and to those requiring certain
technical information, but also to the general public, whose interest in
this entrancing subject may be simply that of pleasure in the purchase,
possession, or collection of precious stones, or even in the mere
examination of them through the plate-glass of a jeweller's window.
JOHN MASTIN.
TOTLEY BROOK,
NEAR SHEFFIELD.
_June 1911._
THE CHEMISTRY, PROPERTIES AND TESTS OF PRECIOUS STONES
CHAPTER I.
INTRODUCTORY.
What constitutes a precious stone is the question which, at the onset,
rises in the mind, and this question, simple as it seems, is one by no
means easy to answer, since what may be considered precious at one time,
may cease to be so at another.
There are, however, certain minerals which possess distinctive features
in their qualities of hardness, colour, transparency, refractability or
double refractability to light-beams, which qualities place them in an
entirely different class to the minerals of a metallic nature. These
particular and non-metallic minerals, therefore, because of their
comparative rarity, rise pre-eminently above other minerals, and become
actually "precious."
This is, at the same time, but a comparative term, for it will readily
be understood that in the case of a sudden flooding of the market with
one class of stone, even if it should be one hitherto rare and
precious, there would be an equally sudden drop in the intrinsic value
of the jewel to such an extent as perhaps to wipe it out of the category
of precious stones. For instance, rubies were discovered long before
diamonds; then when diamonds were found these were considered much more
valuable till their abundance made them common, and they became of
little account. Rubies again asserted their position as chief of all
precious stones in value, and in many biblical references rubies are
quoted as being the symbol of the very acme of wealth, such as in
Proverbs, chapter iii., verses 13 and 15, where there are the passages,
"happy is the man that findeth wisdom ... she is more precious than
rubies"--and this, notwithstanding the enormous quantity of them at that
time obtained from the ruby mines of Ophir and Nubia, which were then
the chief sources of wealth.
It will also be remembered that Josephus relates how, at the fall of
Jerusalem, the spoil of gold was so great that Syria was inundated with
it, and the value of gold there quickly dropped to one-half; other
historians, also, speaking of this time, record such a glut of gold,
silver, and jewels in Syria, as made them of little value, which state
continued for some considerable period, till the untold wealth became
ruthlessly and wastefully scattered, when the normal values slowly
reasserted themselves.
Amongst so many varieties of these precious minerals, it cannot be
otherwise than that there should be important differences in their
various characteristics, though for a stone to have the slightest claim
to be classed as "precious" it must conform to several at least of the
following requirements:--It must withstand the action of light without
deterioration of its beauty, lustre, or substance, and it must be of
sufficient hardness to retain its form, purity and lustre under the
actions of warmth, reasonable wear, and the dust which falls upon it
during use; it must not be subject to chemical change, decomposition,
disintegration, or other alteration of its substance under exposure to
atmospheric air; otherwise it is useless for all practical purposes of
adornment or ornamentation.
There are certain other characteristics of these curious minerals which
may be classified briefly, thus:--Some stones owe their beauty to a
wonderful play of colour or fire, due to the action of light, quite
apart from the colour of the stone itself, and of this series the opal
may be taken as a type. In others, this splendid play of colour is
altogether absent, the colour being associated with the stone itself, in
its substance, the charm lying entirely in the superb transparency, the
ruby being taken as an example of this class of stone. Others, again,
have not only colour, but transparency and lustre, as in the coloured
diamonds, whilst the commoner well-known diamonds are extremely rich in
transparency and lustre, the play of light alone showing a considerable
amount of brilliancy and beauty of colour, though the stone itself is
clear. Still others are opaque, or semi-opaque, or practically free from
play of light and from lustre, owing their value and beauty entirely to
their richness of colour.
In all cases the value of the stone cannot be appreciated fully till the
gem is separated from its matrix and polished, and in some cases, such
as in that of the diamond, cut in variously shaped facets, on and
amongst which the light rays have power to play; other stones, such as
the opal, turquoise and the like, are cut or ground in flat,
dome-shaped, or other form, and then merely polished. It frequently
happens that only a small portion of even a large stone is of supreme
value or purity, the cutter often retaining as his perquisite the
smaller pieces and waste. These, if too small for setting, are ground
into powder and used to cut and polish other stones.
Broadly speaking, the greatest claim which a stone can possess in order
to be classed as precious is its rarity. To this may be added public
opinion, which is led for better or worse by the fashion of the moment.
For if the comparatively common amethyst should chance to be made
extraordinarily conspicuous by some society leader, it would at once
step from its humbler position as semi-precious, and rise to the nobler
classification of a truly precious stone, by reason of the demand
created for it, which would, in all probability, absorb the available
stock to rarity; and this despite the more entrancing beauty of the now
rarer stones.
The study of this section of mineralogy is one of intense interest, and
by understanding the nature, environment, chemical composition and the
properties of the stones, possibility of fraud is altogether precluded,
and there is induced in the mind--even of those with whom the study of
precious stones has no part commercially--an intelligent interest in the
sight or association of what might otherwise excite no more than a mere
glance of admiration or curiosity. There is scarcely any form of matter,
be it liquid, solid, or gaseous, but has yielded or is now yielding up
its secrets with more or less freedom to the scientist. By his method of
synthesis (which is the scientific name for putting substances together
in order to form new compounds out of their union) or of analysis (the
decomposing of bodies so as to divide or separate them into substances
of less complexity), particularly the latter, he slowly and surely
breaks down the substances undergoing examination into their various
constituents, reducing these still further till no more reduction is
possible, and he arrives at their elements. From their behaviour during
the many and varied processes through which they have passed he finds
out, with unerring accuracy, the exact proportions of their composition,
and, in many cases, the cause of their origin.
It may be thought that, knowing all this, it is strange that man does
not himself manufacture these rare gems, such as the diamond, but so far
he has only succeeded in making a few of microscopic size, altogether
useless except as scientific curiosities. The manner in which these
minute gems and spurious stones are manufactured, and the methods by
which they may readily be distinguished from real, will be dealt with in
due course.
The natural stones represent the slow chemical action of water, decay,
and association with, or near, other chemical substances or elements,
combined with the action of millions of years of time, and the unceasing
enormous pressure during that time of thousands, perhaps millions, of
tons of earth, rock, and the like, subjected, for a certain portion at
least of that period, to extremes of heat or cold, all of which
determine the nature of the gem. So that only in the earth itself,
under strictly natural conditions, can these rare substances be found at
all in any workable size; therefore they must be sought after
assiduously, with more or less speculative risk.
CHAPTER II.
THE ORIGIN OF PRECIOUS STONES.
Though the origin, formation, composition, characteristics and tests of
each stone will be examined in detail when dealing with the stones
seriatim, it is necessary to enquire into those particulars of origin
which are common to all, in order thoroughly to understand why they
differ from other non-metallic and metallic minerals.
At the very commencement we are faced with a subject on which
mineralogists and geologists are by no means in full agreement, and
there seems just ground for considerable divergence of opinion,
according to the line of argument taken. It is a most remarkable fact
that, precious as are certain stones, they do not (with a few
exceptions) contain any of the rarer metals, such as platinum, gold,
etc., or any of their compounds, but are composed entirely of the common
elements and their derivatives, especially of those elements contained
in the upper crust of the earth, and this notwithstanding the fact that
gems are often found deep down in the earth. This is very significant,
and points to the conclusion that these stones were formed by the slow
percolation of water from the surface through the deeper parts of the
earth, carrying with it, in solution or suspension, the chemical
constituents of the earth's upper crust; time and long-continued
pressure, combined with heat or cold, or perhaps both in turn, doing the
rest, as already mentioned.
The moisture falling in dew and rain becomes acidulated with carbonic
acid, CO_{2} (carbon dioxide), from the combustion and decay of organic
matter, vegetation, and other sources, and this moisture is capable of
dissolving certain calcareous substances, which it takes deep into the
earth, till the time comes when it enters perhaps a division-plane in
some rock, or some such cavity, and is unable to get away. The hollow
becomes filled with water, which is slowly more and more charged with
the salts brought down, till saturated; then super-saturated, so that
the salts become precipitated, or perhaps crystallised out, maybe by the
presence of more or other salts, or by a change in temperature. These
crystals then become packed hard by further supplies and pressure, till
eventually, after the lapse of ages, a natural gem is found, _exactly
filling_ the cavity, and is a precious find in many cases.
If now we try to find its analogy in chemistry, and for a moment
consider the curious behaviour of some well-known salts, under different
conditions of temperature, what is taking place underground ceases to be
mysterious and becomes readily intelligible.
Perhaps the best salt for the purpose, and one easy to obtain for
experiment, is the sulphate of sodium--known also as Glauber's Salt.
It is in large, colourless prisms, which may soon be dissolved in about
three parts of water, so long as the water does not exceed 60° F., and
at this temperature a super-saturated solution may easily be made. But
if the water is heated the salt then becomes more and more insoluble as
the temperature increases, till it is completely insoluble.
If a super-saturated solution of this Glauber's Salt is made in a glass,
at ordinary atmospheric temperature, and into this cold solution,
without heating, is dropped a small crystal of the same salt, there will
be caused a rise in temperature, and the whole will then crystallise out
quite suddenly; the water will be absorbed, and the whole will solidify
into a mass which exactly fits the inner contour of the vessel.
We have now formed what _might_ be a precious stone, and no doubt would
be, if continuous pressure could be applied to it for perhaps a few
thousand years; at any rate, the formation of a natural jewel is not
greatly different, and after being subjected for a period, extending to
ages, to the washings of moisture, the contact of its containing bed
(its later matrix), the action of the changes in the temperature of the
earth in its vicinity, it emerges by volcanic eruption, earthquake,
landslip and the like, or is discovered as a rare and valuable specimen
of some simple compound of earth-crust and water, as simple as Glauber's
Salt, or as the pure crystallized carbon.
It is also curious to note that in some cases the stones have not been
caused by aqueous deposit in an already existing hollow, but the aqueous
infusion has acted on a portion of the rock on which it rested,
absorbing the rock, and, as it were, replacing it by its own substance.
This is evidenced in cases where gems have been found encrusted on their
matrix, which latter was being slowly transformed to the character of
the jewel encrusted, or "scabbed" on it.
The character of the matrix is also in a great measure the cause of the
variety of the stone, for it is obvious that the same salt-charged
aqueous solution which undergoes change in and on ironstone would result
in an entirely different product from that resting on or embedded in
silica.
Following out the explanation of the aqueous solution, in which the
earth-crust constituents are secreted, we find that the rarer and more
precious metals do not generally enter into the composition of precious
stones--which fact may advisedly be repeated. It is, of course, to be
expected that beryllium will be found in the emerald, since it is under
the species beryl, and zirconium in zircon; but such instances are the
exception, and we may well wonder at the actions of the infinite powers
of nature, when we reflect that the rarest, costliest and most beautiful
of all precious stones are the simplest in their constituents.
Thus we find the diamond standing unique amongst all gems in being
composed of one element only--carbon--being pure crystallised carbon; a
different form from graphite, it is true, but, nevertheless, pure carbon
and nothing else. Therefore, from its chemical, as well as from its
commercial aspect, the diamond stands alone as the most important of
gems.
The next in simplicity, whilst being the most costly of all, is the
ruby, and with this may be classed the blue sapphire, seeing that their
chemical constituents are exactly the same, the difference being one of
colour only. These have two elements, oxygen and aluminium, which
important constituents appear also in other stones, but this example is
sufficient to prove their simplicity of origin.
Another unique stone is the turquoise, in that it is the only rare gem
essentially containing a great proportion of water, which renders it
easily liable to destruction, as we shall see later. It is a combination
of alumina, water, and phosphoric acid, and is also unique in being the
only known valuable stone containing a phosphate.
Turning to the silica series, we again find a number of gems with two
elements only, silica--an important constituent of the earth's
crust--and oxygen--an important constituent of atmospheric air. In this
group may be mentioned the opal, amethyst, agate, rock-crystal, and the
like, as the best known examples, whilst oxygen appears also mostly in
the form of oxides, in chrysoberyl, spinel, and the like. This silica
group is extremely interesting, for in it, with the exception of the
tourmaline and a few others, the composition of the gems is very simple,
and we find in this group such stones as the chrysolite, several
varieties of topaz, the garnet, emerald, etc., etc.
Malachite and similar stones are more ornamental than precious, though
they come in the category of precious stones. These are the carbonate
series, containing much carbonic acid, and, as may be expected, a
considerable proportion of water in their composition, which water can,
of course, be dispelled by the application of heat, but to the
destruction of the stone.
From all this will be seen how strong is the theory of aqueous
percolation, for, given time and pressure, water charged with
earth-crust constituents appears to be the origin of the formation of
all precious stones; and all the precious stones known have, when
analysed, been found to be almost exclusively composed of
upper-earth-crust constituents; the other compounds which certain stones
contain may, in all cases, be traced to their matrix, or to their
geological or mineralogical situation.
In contradistinction to this, the essentially underground liquids, with
time and pressure, form metallic minerals and mineralise the rocks,
instead of forming gems.
Thus we see that in a different class of minerals--compounds of metals
with the sulphates, such as sulphuric acid and compounds; also those
containing the metallic sulphides; in cases where the metalliferous ores
or the metallic elements enter into composition with the
halogens--bromine, chlorine, fluorine, and iodine--in all these,
precious stones are comparatively common, but the stones of these groups
are invariably those used for decorative or ornamental purposes, and
true "gems" are entirely absent.
It would therefore appear that though metallic minerals, as already
mentioned, are formed by the action of essentially _underground_
chemically-charged water--combined with ages of time and long-continued
pressure, rocks and earth being transformed into metalliferous ores by
the same means--precious stones (or that portion of them ranking as
jewels or gems) must on the contrary be wholly, or almost wholly,
composed of _upper_-earth-crust materials, carried deep down by water,
and subjected to the action of the same time and pressure; the simpler
the compound, the more perfect and important the result, as seen in the
diamond, the ruby, and the like.
CHAPTER III.
PHYSICAL PROPERTIES.
A--CRYSTALLINE STRUCTURE.
Before proceeding to the study of precious stones as individual gems,
certain physical properties common to all must be discussed, in order to
bring the gems into separate classes, not only because of some chemical
uniformity, but also because of the unity which exists between their
physical formation and properties.
The first consideration, therefore, may advisedly be that of their
crystals, since their crystalline structure forms a ready means for the
classification of stones, and indeed for that of a multitudinous variety
of substances.
It is one of the many marvellous phenomena of nature that mineral, as
well as many vegetable and animal substances, on entering into a state
of solidity, take upon themselves a definite form called a crystal.
These crystals build themselves round an axis or axes with wonderful
regularity, and it has been found, speaking broadly, that the same
substance gives the same crystal, no matter how its character may be
altered by colour or other means. Even when mixed with other
crystallisable substances, the resulting crystals may partake of the two
varieties and become a sort of composite, yet to the physicist they are
read like an open book, and when separated by analysis they at once
revert to their original form. On this property the analyst depends
largely for his results, for in such matters as food adulteration, etc.,
the microscope unerringly reveals impurities by means of the crystals
alone, apart from other evidences.
It is most curious, too, to note that no matter how large a crystal may
be, when reduced even to small size it will be found that the crystals
are still of the same shape. If this process is taken still further, and
the substance is ground to the finest impalpable powder, as fine as
floating dust, when placed under the microscope each speck, though
perhaps invisible to the naked eye, will be seen a perfect crystal, of
the identical shape as that from which it came, one so large maybe that
its planes and angles might have been measured and defined by rule and
compass. This shows how impossible it is to alter the shape of a
crystal. We may dissolve it, pour the solution into any shaped vessel or
mould we desire, recrystallise it and obtain a solid sphere, triangle,
square, or any other form; it is also possible, in many cases, to
squeeze the crystal by pressure into a tablet, or any form we choose,
but in each case we have merely altered the _arrangement_ of the
crystals, so as to produce a differently shaped _mass_, the crystals
themselves remaining individually as before. Such can be said to be one
of the laws of crystals, and as it is found that every substance has its
own form of crystal, a science, or branch of mineralogy, has arisen,
called "crystallography," and out of the conglomeration of confused
forms there have been evolved certain rules of comparison by which all
known crystals may be classed in certain groups.
This is not so laborious a matter as would appear, for if we take a
substance which crystallises in a cube we find it is possible to draw
nine symmetrical planes, these being called "planes of symmetry," the
intersections of one or more of which planes being called "axes of
symmetry." So that in the nine planes of symmetry of the cube we get
three axes, each running through to the opposite side of the cube. One
will be through the centre of a face to the opposite face; a second will
be through the centre of one edge diagonally; the third will be found in
a line running diagonally from one point to its opposite. On turning the
cube on these three axes--as, for example, a long needle running through
a cube of soap--we shall find that four of the six identical faces of
the cube are exposed to view during each revolution of the cube on the
needle or axis.
These faces are not necessarily, or always, planes, or flat, strictly
speaking, but are often more or less curved, according to the shape of
the crystal, taking certain characteristic forms, such as the square,
various forms of triangles, the rectangle, etc., and though the crystals
may be a combination of several forms, all the faces of any particular
form are similar.
All the crystals at present known exhibit differences in their planes,
axes and lines of symmetry, and on careful comparison many of them are
found to have some features in common; so that when they are sorted out
it is seen that they are capable of being classified into thirty-three
groups. Many of these groups are analogous, so that on analysing them
still further we find that all the known crystals may be classed in six
separate systems according to their planes of symmetry, and all stones
of the same class, no matter what their variety or complexity may be,
show forms of the same group. Beginning with the highest, we have--(1)
the cubic system, with nine planes of symmetry; (2) the hexagonal, with
seven planes; (3) the tetragonal, with five planes; (4) the rhombic,
with three planes; (5) the monoclinic, with one plane; (6) the
triclinic, with no plane of symmetry at all.
In the first, the cubic--called also the isometric, monometric, or
regular--there are, as we have seen, three axes, all at right angles,
all of them being equal.
The second, the hexagonal system--called also the rhombohedral--is
different from the others in having four axes, three of them equal and
in one plane and all at 120° to each other; the fourth axis is not
always equal to these three. It may be, and often is, longer or shorter.
It passes through the intersecting point of the three others, and is
perpendicular or at right angles to them.
The third of the six systems enumerated above, the tetragonal--or the
quadratic, square prismatic, dimetric, or pyramidal--system has three
axes like the cubic, but, in this case, though they are all at right
angles, two only of them are equal, the third, consequently, unequal.
The vertical or principal axis is often much longer or shorter in this
group, but the other two are always equal and lie in the horizontal
plane, at right angles to each other, and at right angles to the
vertical axis.
The fourth system, the rhombic--or orthorhombic, or prismatic, or
trimetric--has, like the tetragonal, three axes; but in this case, none
of them are equal, though the two lateral axes are at right angles to
each other, and to the vertical axis, which may vary in length, more so
even than the other two.
The fifth, the monoclinic--or clinorhombic, monosymmetric, or
oblique--system, has also three axes, all of them unequal. The two
lateral axes are at right angles to each other, but the principal or
vertical axis, which passes through the point of intersection of the two
lateral axes, is only at right angles to one of them.
In the sixth and last system, the triclinic--or anorthic, or
asymmetric--the axes are again three, but in this case, none of them are
equal and none at right angles.
It is difficult to explain these various systems without drawings, and
the foregoing may seem unnecessarily technical. It is, however,
essential that these particulars should be clearly stated in order
thoroughly to understand how stones, especially uncut stones, are
classified. These various groups must also be referred to when dealing
with the action of light and other matters, for in one or other of them
most stones are placed, notwithstanding great differences in hue and
character; thus all stones exhibiting the same crystalline structure as
the diamond are placed in the same group. Further, when the methods of
testing come to be dealt with, it will be seen that these particulars of
grouping form a certain means of testing stones and of distinguishing
spurious from real. For if a stone is offered as a real gem (the true
stone being known to lie in the highest or cubic system), it follows
that should examination prove the stone to be in the sixth system, then,
no matter how coloured or cut, no matter how perfect the imitation, the
test of its crystalline structure stamps it readily as false beyond all
shadow of doubt--for as we have seen, no human means have as yet been
forthcoming by which the crystals can be changed in form, only in
arrangement, for a diamond crystal _is_ a diamond crystal, be it in a
large mass, like the brightest and largest gem so far discovered--the
great Cullinan diamond--or the tiniest grain of microscopic
diamond-dust, and so on with all precious stones. So that in future
references, to avoid repetition, these groups will be referred to as
group 1, 2, and so on, as detailed here.
CHAPTER IV.
PHYSICAL PROPERTIES.
B--CLEAVAGE.
By cleavage is meant the manner in which minerals separate or split off
with regularity. The difference between a break or fracture and a
"cleave," is that the former may be anywhere throughout the substance of
the broken body, with an extremely remote chance of another fracture
being identical in form, whereas in the latter, when a body is
"cleaved," the fractured part is more readily severed, and usually takes
a similar if not an actually identical form in the divided surface of
each piece severed. Thus we find a piece of wood may be "broken" or
"chopped" when fractured across the grain, no two fractured edges being
alike; but, strictly speaking, we only "cleave" wood when we "split" it
with the grain, or, in scientific language, along the line of cleavage,
and then we find many pieces with their divided surfaces identical. So
that when wood is "broken," or "chopped," we obtain pieces of any width
or thickness, with no manner of regularity of fracture, but when
"cleaved," we obtain strips which are often perfectly parallel, that is,
of equal thickness throughout their whole length, and of such uniformity
of surface that it is difficult or even impossible to distinguish one
strip from another. Advantage is taken of these lines of cleavage to
procure long and extremely thin even strips from trees of the willow
variety for such trades as basket-making.
The same effect is seen in house-coal, which may easily be split the way
of the grain (on the lines of cleavage), but is much more difficult and
requires greater force to break across the grain. Rocks also show
distinct lines of cleavage, and are more readily split one way than
another, the line of cleavage or stratum of break being at any angle and
not necessarily parallel to its bed. A striking example of this is seen
in slate, which may be split in plates, or laminæ, with great facility,
though this property is the result of the pressure to which the rock has
been for ages subjected, which has caused a change in the molecules,
rather than by "cleavage" as the term is strictly understood, and as
existing in minerals. Mica is also another example of laminated
cleavage, for given care, and a thin, fine knife to divide the plates,
this mineral may be "cleaved" to such remarkably thin sheets as to be
unable to sustain the most delicate touch without shattering.
These are well-known examples of simple cleavage, in one definite
direction, though in many instances there are several forms and
directions of cleavage, but even in these there is generally one part or
line in and on which cleavage will take place much more readily than on
the others, these planes or lines also showing different properties and
angular characters, which, no matter how much fractured, always remain
the same. It is this "cleavage" which causes a crystal to reproduce
itself exactly, as explained in the last chapter, showing its parent
form, shape and characteristics with microscopic perfection, but more
and more in miniature as its size is reduced.
This may clearly be seen by taking a very small quantity of such a
substance as chlorate of potash. If a crystal of this is examined under
a magnifying glass till its crystalline form and structure are familiar,
and it is then placed in a test-tube and gently heated, cleavage will at
once be evident. With a little crackling, the chlorate splits itself
into many crystals along its chief lines of cleavage (called the
cleavage planes), every one of which crystals showing under the
microscope the identical form and characteristics of the larger crystal
from which it came.
The cleavage of minerals must, therefore, be considered as a part of
their crystalline structure, since this is caused by cleavage, so that
both cleavage and crystalline structure should be considered together.
Thus we see that given an unchangeable crystal with cleavage planes
evident, it is possible easily to reproduce the same form over and over
again by splitting, whereas by simply breaking, the form of the crystal
would be lost; just as a rhomb of Iceland spar might be sawn or broken
across the middle and its form lost, although this would really be more
apparent than real, since it would be an alteration in the mass and not
in the shape of each individual crystal. And given further cleavage, by
time or a sudden breaking down, even the mass, as mass, would eventually
become split into smaller but perfect rhombs.
Much skill is, therefore, required in cutting and fashioning a precious
stone, otherwise the gem may be ruined at the onset, for it will only
divide along its lines of cleavage, and any mistake in deciding upon
these, would "break," not "split" the stone, and destroy the beauty of
its crystalline structure. An example of this was specially seen in the
great Cullinan diamond, the splitting of which was perhaps the most
thrilling moment in the history of precious stones.[A] The value of the
enormous crystal was almost beyond computation, but it had a flaw in the
centre, and in order to cut out this flaw it was necessary to divide the
stone into two pieces. The planes of cleavage were worked out, the
diamond was sawn a little, when the operator, acknowledged to be the
greatest living expert, inserted a knife in the saw-mark, and with the
second blow of a steel rod, the marvellous stone parted precisely as
intended, cutting the flaw exactly in two, leaving half of it on the
outside of each divided portion. The slightest miscalculation would have
meant enormous loss, if not ruin, to the stone, but the greatest feat
the world has ever known in the splitting of a priceless diamond was
accomplished successfully by this skilful expert in an Amsterdam
workroom in February, 1908. Some idea of the risk involved may be
gathered from the fact that this stone, the largest ever discovered, in
the rough weighed nearly 3,254 carats, its value being almost anything
one cared to state--incalculable.
[Footnote A: The hammer and knife used in cutting the diamond, the two
largest pieces of which are now called "The Stars of Africa," together
with a model of the great uncut stone, are in the Tower of London
amongst the Regalia.]
These cleavage planes help considerably in the bringing of the stone to
shape, for in a broad sense, a finished cut stone may be said to be in
the form in which its cleavages bring it. Particularly is this seen in
the diamond "brilliant," which plainly evidences the four cleavage
planes. These cleavage planes and their number are a simple means of
identification of precious stones, though those possessing distinct and
ready cleavages are extremely liable to "start" or "split" on these
planes by extremes of heat and cold, accidental blows, sudden shocks and
the like.
In stones possessing certain crystalline structure, the cleavage planes
are the readiest, often the only, means of identification, especially
when the stones are chemically coloured to imitate a more valuable
stone. In such cases the cleavage of one stone is often of paramount
importance in testing the cleavage of another, as is seen in the
perfection of the cleavage planes of calcite, which is used in the
polariscope.
It sometimes happens, however, that false conditions arise, such as in
substances which are of no form or shape, and are in all respects and
directions without regular structure and show no crystallisation even in
the minutest particles; these are called amorphous. Such a condition
sometimes enters wholly or partially into the crystalline structure, and
the mineral loses its true form, possessing instead the form of
crystals, but without a crystalline structure. It is then called a
pseudomorph, which is a term applied to any mineral which, instead of
having the form it should possess, shows the form of something which has
altered its structure completely, and then disappeared. For instance:
very often, in a certain cavity, fluorspar has existed originally, but,
through some chemical means, has been slowly changed to quartz, so
that, as crystals cannot be changed in shape, we find quartz
existing--undeniably quartz--yet possessing the crystals of fluorspar;
therefore the quartz becomes a pseudomorph, the condition being an
example of what is termed pseudomorphism. The actual cause of this
curious chemical change or substitution is not known with certainty, but
it is interesting to note the conditions in which such changes do occur.
It is found that in some cases, the matrix of a certain shaped crystal
may, after the crystal is dissolved or taken away, become filled by some
other and foreign substance, perhaps in liquid form; or a crystalline
substance may become coated or "invested" by another foreign substance,
which thus takes its shape; or actual chemical change takes place by
means of an incoming substance which slowly alters the original
substance, so that eventually each is false and both become
pseudomorphs. This curious change often takes place with precious
stones, as well as with other minerals, and to such an extent that it
sometimes becomes difficult to say what the stone ought really to be
called.
Pseudomorphs are, however, comparatively easy of isolation and
detection, being more or less rounded in their crystalline form, instead
of having sharp, well-defined angles and edges; their surfaces also are
not good. These stones are of little value, except in the specially
curious examples, when they become rare more by reason of their
curiosity than by their utility as gems.
Some also show cleavage planes of two or more systems, and others show
a crystalline structure comprised of several systems. Thus calcspar is
in the 2nd, or hexagonal, whilst aragonite is in the 4th, the rhombic,
system, yet both are the same substance, viz.:--carbonate of lime. Such
a condition is called dimorphism; those minerals which crystallise in
three systems are said to be trimorphous. Those in a number of systems
are polymorphous, and of these sulphur may be taken as an example, since
it possesses thirty or more modifications of its crystalline structure,
though some authorities eliminate nearly all these, and, since it is
most frequently in either the 4th (rhombic) or the 5th (monoclinic)
systems, consider it as an example of dimorphism, rather than
polymorphism.
These varieties of cleavage affect the character, beauty and usefulness
of the stone to a remarkable extent, and at the same time form a means
of ready and certain identification and classification.
CHAPTER V.
PHYSICAL PROPERTIES.
C--LIGHT.
Probably the most important of the many important physical properties
possessed by precious stones are those of light and its effects, for to
these all known gems owe their beauty, if not actual fascination.
When light strikes a cut or polished stone, one or more of the following
effects are observed:--it may be transmitted through the stone,
diaphaneity, as it is called; it may produce single or double
refraction, or polarisation; if reflected, it may produce lustre or
colour; or it may produce phosphorescence; so that light may be (1)
transmitted; (2) reflected; or produce (3) phosphorescence.
(1) TRANSMISSION.--In transmitted light we have, as stated above, single
or double refraction, polarisation, and diaphaneity.
To the quality of _refraction_ is due one of the chief charms of certain
precious stones. It is not necessary to explain here what refraction is,
for everyone will be familiar with the refractive property of a
light-beam when passing through a medium denser than atmospheric air. It
will be quite sufficient to say that all the rays are not equal in
refractive power in all substances, so that the middle of the spectrum
is generally selected as the mean for indexing purposes.
It will be seen that the stones in the 1st, or cubic system, show single
refraction, whereas those of all other systems show double refraction;
thus, light, in passing through their substance, is deviated, part of it
going one way, the other portion going in another direction--that is, at
a slightly different angle--so that this property alone will isolate
readily all gems belonging to the 1st system.
A well-known simple experiment in physics shows this clearly. A mark on
a card or paper is viewed through a piece of double-refracting spar
(Iceland spar or clear calcite), when the mark is doubled and two
appear. On rotating this rhomb of spar, one of these marks is seen to
revolve round the other, which remains stationary, the moving mark
passing further from the centre in places. When the spar is cut and used
in a certain direction, we see but one mark, and such a position is
called its optical axis.
_Polarisation_ is when certain crystals possessing double refraction
have the power of changing light, giving it the appearance of poles
which have different properties, and the polariscope is an instrument in
which are placed pieces of double-refracting (Iceland) spar, so that all
light passing through will be polarised.
Since only crystals possessing the property of double refraction show
polarisation, it follows that those of the 1st, or cubic system--in
which the diamond stands a prominent example--fail to become polarised,
so that when such a stone is placed in the polariscope and rotated, it
fails _at every point_ to transmit light, which a double-refracting gem
allows to pass except when its optical axis is placed in the axis of the
polariscope, but this will be dealt with more fully when the methods of
testing the stones come to be considered.
_Diaphaneity_, or the power of transmitting light:--some rather fine
trade distinctions are drawn between the stones in this class, technical
distinctions made specially for purposes of classification, thus:--a
"non-diaphanous" stone is one which is quite opaque, no light of any
kind passing through its substance; a "diaphanous" stone is one which is
altogether transparent; "semi-diaphanous" means one not altogether
transparent, and sometimes called "sub-transparent." A "translucent"
stone is one in which, though light passes through its substance, sight
is not possible through it; whilst in a "sub-translucent" stone, light
passes through it, but only in a small degree.
The second physical property of light is seen in those stones which owe
their beauty or value to REFLECTION: this again may be dependent on
Lustre, or Colour.
~Lustre.~--This is an important characteristic due to reflection, and of
which there are six varieties:--([alpha]) adamantine (which some
authorities, experts and merchants subdivide as detailed below);
([beta]) pearly; ([gamma]) silky; ([delta]) resinous; ([epsilon])
vitreous; ([zeta]) metallic. These may be described:--
([alpha]) Adamantine, or the peculiar lustre of the diamond, so called
from the lustre of adamantine spar, which is a form of corundum (as is
emery) with a diamond-like lustre, the hard powder of which is used in
polishing diamonds. It is almost pure anhydrous alumina (Al_{2}O_{3})
and is, roughly, four times as heavy as water. The lustre of this is the
true "adamantine," or diamond, brilliancy, and the other and impure
divisions of this particular lustre are: _splendent_, when objects are
reflected perfectly, but of a lower scale of perfection than the true
"adamantine" standard, which is absolutely flawless. When still lower,
and the reflection, though maybe fairly good, is somewhat "fuzzy," or is
confused or out of focus, it is then merely _shining_; when still less
distinct, and no trace of actual reflection is possible (by which is
meant that no object can be reproduced in any way to define it, as it
could be defined in the reflection from still water or the surface of a
mirror, even though imperfectly) the stone is then said to _glint_ or
_glisten_. When too low in the scale even to glisten, merely showing a
feeble lustre now and again as the light is reflected from its surface
in points which vary with the angle of light, the stone is then said to
be _glimmering_. Below this, the definitions of lustre do not go, as
such stones are said to be _lustreless_.
([beta]) Pearly, as its name implies, is the lustre of a pearl.
([gamma]) Silky, possessing the sheen of silk, hence its name.
([delta]) Resinous, also explanatory in its name; amber and the like
come in this variety.
([epsilon]) Vitreous. This also explains itself, being of the lustre of
glass, quartz, etc.; some experts subdividing this for greater defining
accuracy into the "sub-vitreous" or lower type, for all but perfect
specimens.
([zeta]) Metallic or Sub-metallic. The former when the lustre is perfect
as in gold; the latter when the stones possess the less true lustre of
copper.
~Colour.~--Colour is an effect entirely dependent upon light, for in the
total absence of light, such as in black darkness, objects are
altogether invisible to the normal human eye. In daylight, also, certain
objects reflect so few vibrations of light, or none, that they appear
grey, black, or jet-black; whilst those which reflect all the rays of
which light is composed, and in the same number of vibrations, appear
white. Between these two extremes of _none_ and _all_ we find a
wonderful play and variety of colour, as some gems allow the red rays
only to pass and therefore appear red; others allow the blue rays only
and these appear blue, and so on, through all the shades, combinations
and varieties of the colours of which light is composed, as revealed by
the prism. But this is so important a matter that it demands a chapter
to itself.
The third physical property of light, PHOSPHORESCENCE, is the property
possessed by certain gems and minerals of becoming phosphorescent on
being rubbed, or on having their temperature raised by this or other
means.
It is difficult to say exactly whether this is due to the heat, the
friction, or to electricity. Perhaps two or all of these may be the
cause, for electricity is developed in some gems--such as the topaz--by
heat, and heat by electricity, and phosphorescence developed by both.
For example, if we rub together some pulverised fluorspar in the dark,
or raise its temperature by the direct application of heat, such as from
a hot or warm iron, or a heated wire, we at once obtain excellent
phosphorescence. Common quartz, rubbed against a second piece of the
same quartz in the dark, becomes highly phosphorescent. Certain gems,
also, when merely exposed to light--sunlight for preference--then taken
into a darkened room, will glow for a short time. The diamond is one of
the best examples of this kind of phosphorescence, for if exposed to
sunlight for a while, then covered and rapidly taken into black
darkness, it will emit a curious phosphorescent glow for from one to ten
seconds; the purer the stone, the longer, clearer and brighter the
result.
CHAPTER VI.
PHYSICAL PROPERTIES.
D--COLOUR.
Colour is one of the most wonderful effects in nature. It is an
attribute of light and is not a part of the object which appears to be
coloured; though all objects, by their chemical or physical composition,
determine the number and variety of vibrations passed on or returned to
the eye, thus fixing their own individual colours.
We have also seen that if an _equal_ light-beam becomes obstructed in
its passage by some substance which is denser than atmospheric air, it
will become altered in its direction by refraction or reflection, and
polarised, each side or pole having different properties.
Polarised light cannot be made again to pass in a certain direction
through the crystal which has polarised it; nor can it again be
reflected at a particular angle; so that in double-refracting crystals,
these two poles, or polarised beams, are different in colour, some
stones being opaque to one beam but not to the other, whilst some are
opaque to both.
This curious phenomenon, with this brief, though somewhat technical
explanation, shows the cause of many of the great charms in precious
stones, for when viewed at one angle they appear of a definite colour,
whilst at another angle they are just as decided in their colour, which
is then entirely different; and as these angles change as the eye
glances on various facets, the stone assumes a marvellous wealth of the
most brilliant and intense colour of kaleidoscopic variety, even in a
stone which may itself be absolutely clear or colourless to ordinary
light.
Such an effect is called pleochroism, and crystals which show variations
in their colour when viewed from different angles, or by transmitted
light, are called pleochroic, or pleochromatic--from two Greek words
signifying "to colour more." To aid in the examination of this
wonderfully beautiful property possessed by precious stones, a little
instrument has been invented called the dichroscope, its name showing
its Greek derivation, and meaning--"to see colour twice" (twice, colour,
to see). It is often a part of a polariscope; frequently a part also of
the polarising attachment to the microscope, and is so simple and
ingenious as to deserve detailed explanation.
In a small, brass tube is fixed a double-image prism of calcite or
Iceland spar, which has been achromatised--that is, clear, devoid of
colour--and is therefore capable of transmitting light without showing
any prismatic effect, or allowing the least trace of any except the
clear light-beam to pass through. At one end of this tube there is a
tiny square hole, the opposite end carrying a small convex lens, of such
a strength or focus as to show the square hole in true focus, that is,
with perfectly sharp definition, even up to the corners of the square.
On looking through the tube, the square hole is duplicated, two squares
being seen. The colours of a gem are tested by the stone being put in
front of this square, when the two colours are seen quite distinctly.
Not only is this a simple means of judging colour, but it enables a
stone to be classified readily. For if the dichroscope shows two images
of _the same_ colour, then it may possibly be a carbuncle, or a diamond,
as the case may be--for single-refracting stones, of the first or cubic
system, show two images of _the same_ colour. But if these two colours
are different, then it must be a double-refracting stone, and according
to the particular colours seen, so is the stone classified, for each
stone has its own identical colour or colours when viewed through this
small but useful instrument.
How clear and distinct are these changes may be viewed without it in
substances strongly dichroic; for instance, if common mica is viewed in
one direction, it is transparent as polished plate-glass, whilst at
another angle, it is totally opaque. Chloride of palladium also is
blood-red when viewed parallel to its axis, and transversely, it is a
remarkably bright green. The beryl also, is sea-green one way and a
beautiful blue another; the yellow chrysoberyl is brown one way and
yellow with a greenish cast when viewed another way. The pink topaz
shows rose-colour in one direction and yellow in another. These are
perhaps the most striking examples, and are mostly self-evident to the
naked eye, whilst in other cases, the changes are so delicate that the
instrument must be used to give certainty; some again show changes of
colour as the stone is revolved in the dichroscope, or the instrument
revolved round the stone.
Some stones, such as the opal, split up the light-beams as does a
prism, and show a wonderful exhibition of prismatic colour, which is
technically known as a "play of colour." The descriptive term
"opalescence" is self-suggesting as to its origin, which is the "noble"
or "precious" opal; this radiates brilliant and rapidly changing
iridescent reflections of blue, green, yellow and red, all blending
with, and coming out of, a curious silky and milky whiteness, which is
altogether characteristic. The moonstone is another example of this
peculiar feature which is possessed in a more or less degree by all the
stones in the class of pellucid jewels, but no stone or gem can in any
way even rival the curious mixture of opaqueness, translucency,
silkiness, milkiness, fire, and the steadfast changeable and prismatic
brilliance of colour of the precious opal. The other six varieties of
opal are much inferior in their strange mixture of these anomalies of
light and colour. Given in order of value, we have as the second, the
"fire" opal with a red reflection, and, as a rule, that only. The third
in value is the "common" opal, with the colours of green, red, white and
yellow, but this is easily distinguishable from the "noble" or
"precious" variety in that the common opal does not possess that
wonderful "play" of colour. The fourth variety is called the
"semi-opal," which is really like the third variety, the "common," but
of a poorer quality and more opaque. The fifth variety in order of
value, is that known as the "hydrophane," which has an interesting
characteristic in becoming transparent when immersed in water, and only
then. The sixth is the "hyalite," which has but a glassy or vitreous
lustre, and is found almost exclusively in the form of globules, or
clusters of globules, somewhat after the form and size of bunches of
grapes; hence the name "botryoidal" is often applied to this variety.
The last and commonest of all the seven varieties of opal is somewhat
after the shape of a kidney (reniform), or other irregular shape,
occasionally almost transparent, but more often somewhat translucent,
and very often opaque. This seventh class is called "menilite," being
really an opaline form of quartz, originally found at Menilmontant,
hence its name (_Menil_, and Greek _lithos_, stone). It is a curious
blue on the exterior of the stone, brown inside.
History records many magnificent and valuable opals, not the least of
which was that of Nonius, who declined to give it to Mark Antony,
choosing exile rather than part with so rare a jewel, which Pliny
describes as being existent in his day, and of a value which, in present
English computation, would exceed one hundred thousand pounds.
Many other stones possess one or more properties of the opal, and are
therefore considered more or less opalescent. This "play of colour" and
"opalescence," must not be confused with "change of colour." The two
first appear mostly in spots and in brilliant points or flashes of
coloured light, or "fire" as it is termed. This fire is constantly on
the move, or "playing," whereas "change of colour," though not greatly
dissimilar, is when the fire merely travels over broader surfaces, each
colour remaining constant, such as when directly moving the stone, or
turning it, when the broad mass of coloured light slowly changes,
usually to its complementary. Thus in this class of stone, subject to
"change of colour," a green light is usually followed by its
complementary, red, yellow by purple, blue by orange, green by brown,
orange by grey, purple by broken green, with all the intermediary shades
of each.
Thus when the line of sight is altered, or the stone moved, never
otherwise, the colours chase one another over the surface of the gem,
and mostly in broad splashes; but in those gems possessing "play of
colour," strictly speaking, whilst the stone itself remains perfectly
still, and the sight is fixed unwaveringly upon it, the pulsations of
the blood in the eyes, with the natural movements of the eyes and
eyelids, even in a fixed, steady glance, are quite sufficient to create
in the stone a display of sparks and splashes of beautiful fiery light
and colour at every tremor.
The term "iridescence" is used when the display of colour is seen on the
surface, rather than coming out of the stone itself. The cause of this
is a natural, or in some cases an accidental, breaking of the surface of
the stone into numerous cobweb-like cracks; these are often of
microscopic fineness, only perceptible under moderately high powers.
Nevertheless they are quite sufficient to interfere with and refract the
light rays and to split them up prismatically. In some inferior stones
this same effect is caused or obtained by the application of a gentle
heat, immersion in chemicals, subjection to "X rays" and other strong
electric influence, and in many other ways. As a result, the stone is
very slightly expanded, and as the molecules separate, there appear on
the surface thousands, perhaps millions, of microscopic fissures running
at all angles, so that no matter from what position the stone may be
viewed, a great number of these fissures are certain to split up the
light into prismatic colours causing brilliant iridescence. Similar
fissures may often be seen with the naked eye on glass, especially if
scorched or cooled too rapidly (chilled), and on the surface of clear
spar and mica, their effects being of extreme interest, from a colour
point of view, at least.
CHAPTER VII.
PHYSICAL PROPERTIES.
E--HARDNESS.
Hardness is perhaps one of the most important features in a stone,
especially those of the "gem" series, for no matter how colour, lustre,
general beauty and even rarity may entitle a stone to the designation
"precious," unless it possesses great hardness it cannot be used as a
gem or jewel.
Consequently, the hardness of jewels is a matter of no small importance,
and by dint of indefatigable research, in tests and comparison, all
known precious stones have been classified in various scales or degrees
of hardness. The most popular and reliable table is that of Mohs, in
which he takes talc as the softest of the rarer minerals and classes
this as No. 1; from that he goes by gradual steps to the diamond, the
hardest of the stones, which he calls No. 10, and between these two all
other gems are placed. Here is given a complete list of Mohs's
arrangement of stones, according to their hardness, beginning at No. 1,
thus:--
Talc 1
Rock salt 2
Amber 2-1/2
Calcite 3
Malachite 3-1/2
Jet 3-1/2
Fluorspar 4
Apatite 5
Dioptase 5
Kyanite (various) 5-7
Haüynite 5-1/2
Hæmatite 5-1/2
Lapis lazuli 5-1/2
Moldavite (various) 5-1/2-6-1/2
Rhodonite 5-1/2-6-1/2
Obsidian 5-1/2
Sphene 5-1/2
Opal (various) 5-1/2-6-1/2
Nephrite 5-3/4
Chrysolite 6-7
Felspar 6
Adularia 6
Amazon stone 6
Diopside 6
Iron pyrites 6
Labradorite 6
Turquoise 6
Spodumene 6-1/2-7
The Chalcedony group which embraces the Agate,
Carnelian, etc. 6-1/2
Demantoid 6-1/2
Epidote 6-1/2
Idocrase 6-1/2
Garnets (see also "Red Garnets" below) 6-1/2-7-1/2
Axinite 6-3/4
Jadeite 6-3/4
Quartz, including Rock-crystal, Amethyst, Jasper,
Chrysoprase Citrine, etc. 7
Jade 7
Dichorite (water sapphire) 7-7-1/2
Cordierite 7-1/4
Red Garnets (see also Garnets above) 7-1/4
Tourmaline 7-1/4
Andalusite 7-1/2
Euclase 7-1/2
Staurolite 7-1/2
Zircon 7-1/2
Emerald, Aquamarine, or Beryl 7-3/4
Phenakite 7-3/4
Spinel 8
Topaz 8
Chrysoberyl 8-1/2
The Corundum group embracing the Ruby, Sapphire, etc. 9
Diamond 10
(See also list of stones, arranged in their respective colours, in
Chapter XII.)
The method of testing is very simple. A representative selection of the
above stones, each with a sharp edge, is kept for the purpose of
scratching and being scratched, and those usually set apart for tests in
the various groups, are as follows:--
1 Talc
2 Rock-salt, or Gypsum
3 Calcite
4 Fluorspar
5 Apatite
6 Felspar
7 Quartz
8 Topaz
9 Corundum
10 Diamond
The stone under examination may perhaps first be somewhat roughly
classified by its colour, cleavage, and general shape. One of these
standard stones is then gently rubbed across its surface and then others
of increasingly higher degrees, till no scratch is evident under a
magnifying glass. Thus if quartz ceases to scratch it, but a topaz will
do so, the degree of hardness must lie between 7 and 8. Then we reverse
the process: the stone is passed over the standard, and if both quartz
and topaz are scratched, then the stone is at least equal in hardness to
the topaz, and its classification becomes an easy matter.
Instead of stones, some experts use variously-tempered needles of
different qualities and compositions of iron and steel. For instance, a
finely-tempered ordinary steel needle will cut up to No. 6 stones; one
made of tool steel, up to 7; one of manganese steel, to 7-1/2; one made
of high-speed tool steel, to 8 and 8-1/2, and so on, according to
temper; so that from the scratch which can be made with the finger-nail
on mica, to the hardness of the diamond, which diamond alone will
scratch readily, the stones may be picked out, classified and tested,
with unerring accuracy.
It will thus be seen how impossible it is, even in this one of many
tests, for an expert to be deceived in the purchase of precious stones,
except through gross carelessness--a fault seldom, if ever, met with in
the trade. For example--a piece of rock-crystal, chemically coloured,
and cut to represent a ruby, might appear so like one as to deceive a
novice, but the mere application to its surface of a real ruby, which is
hardness 9, or a No. 9 needle, would reveal too deep or powdery a
scratch; also its possibility of being scratched by a topaz or a No. 8
needle, would alone prove it false, for the corundum group, being harder
than No. 8, could not be scratched by it. So would the expert go down
the scale, the tiny scratches becoming fainter as he descended, because
he would be approaching more nearly the hardness of the stone under
test, till he arrived at the felspar, No. 6, which would be too soft to
scratch it, yet the stone would scratch the felspar, but not zircon or
andalusite, 7-1/2, or topaz, 8, so that his tests would at once classify
the stone as a piece of cut and coloured quartz, thus confirming what he
would, at the first sight, have suspected it to be.
The standard stones themselves are much more certain in results than the
needles, which latter, though well selected and tempered, are not
altogether reliable, especially in the more delicate distinctions of
picking out the hardest of certain stones of the same kind, in which
cases only the expert judge can decide with exactness. Accurate in this
the expert always is, for he judges by the sound and depth of his cut,
and by the amount and quality of the powder, often calling the
microscope to his aid, so that when the decision is made finally, there
is never the least doubt about it.
Rapidly as these tests can be made, they are extremely reliable, and
should the stone be of great value, it is also subjected to other
unerring tests of extreme severity, any one of which would prove it
false, if it chanced to be so, though some stones are manufactured and
coloured so cleverly that to all but the expert judge and experienced
dealer, they would pass well for the genuine.
In Mohs's list it will be seen that several stones vary considerably,
the opal, for instance, having a degree of hardness from 5-1/2 to 6-1/2
inclusive. All stones differ slightly, though almost all may be said to
fit their position in the scale; but in the case of the opal, the
difference shown is partly due to the many varieties of the stone, as
described in the last chapter.
In applying this test of hardness to a cut gem, it will be noticed that
some parts of the same stone seem to scratch more readily than others,
such as on a facet at the side, which is often softer than those nearest
the widest part of the stone, where the claws, which hold it in its
setting, usually come. This portion is called the "girdle," and it is on
these "girdle" facets that the scratches are generally made. This
variation in hardness is mostly caused by cleavage, these cleavage
planes showing a marked, though often but slight, difference in the
scratch, which difference is _felt_ rather than seen. In addition to the
peculiar _feel_ of a cutting scratch, is the _sound_ of it. On a soft
stone being cut by a hard one, little or no sound is heard, but there
will form a plentiful supply of powder, which, on being brushed off,
reveals a more or less deep incision. But as the stones approach one
another in hardness, there will be little powder and a considerable
increase in the noise; for the harder are the stones, cutting and being
cut, the louder will be the sound and the less the powder. An example
of this difference is evident in the cutting of ordinary glass with a
"set" or "glazier's" diamond, and with a nail. If the diamond is held
properly, there will be heard a curious sound like a keen, drawn-out
"kiss," the diamond being considerably harder than the material it cut.
An altogether different sound is that produced by the scratching of
glass with a nail. In this case, the relative difference in hardness
between the two is small, so that the glass can only be scratched and
not "cut" by the nail; it is too hard for that, so the noise is much
greater and becomes a screech. Experience, therefore, makes it possible
to tell to a trifle, at the first contact, of what the stone is
composed, and in which class it should be placed, by the mere "feel" of
the scratch, the depth of it, the amount and kind of powder it leaves,
and above all, by the sound made, which, even in the tiniest scratch, is
quite characteristic.
CHAPTER VIII.
PHYSICAL PROPERTIES.
F--SPECIFIC GRAVITY.
The fixing of the specific gravity of a stone also determines its group
position with regard to weight; its colour and other characteristics
defining the actual stone. This is a safe and very common method of
proving a stone, since its specific gravity does not vary more than a
point or so in different specimens of the same stone. There are several
ways of arriving at this, such as by weighing in balances in the usual
manner, by displacement, and by immersion in liquids the specific
gravity of which are known. Cork is of less specific gravity than water,
therefore it floats on the surface of that liquid, whereas iron, being
heavier, sinks. So that by changing the liquid to one lighter than cork,
the cork will sink in it as does iron in water; in the second instance,
if we change the liquid to one heavier than iron, the iron will float on
it as does cork on water, and exactly as an ordinary flat-iron will
float on quicksilver, bobbing up and down like a cork in a tumbler of
water. If, therefore, solutions of known but varying densities are
compounded, it is possible to tell almost to exactitude the specific
gravity of any stone dropped into them, by the position they assume.
Thus, if we take a solution of pure methylene iodide, which has a
specific gravity of 3.2981, and into this drop a few stones selected
indiscriminately, the effect will be curious: first, some will sink
plump to the bottom like lead; second, some will fall so far quickly,
then remain for a considerable time fairly stationary; third, some will
sink very slowly; fourth, some will be partially immersed, that is, a
portion of their substance being above the surface of the liquid and a
portion covered by it; fifth, some will float on the surface without any
apparent immersion. In the first case, the stones will be much heavier
than 3.2981; in the second, the stones will be about 3.50; in the third
and fourth instances, the stones will be about the same specific gravity
as the liquid, whilst in the fifth, they will be much lighter, and thus
a rough but tolerably accurate isolation may be made.
On certain stones being extracted and placed in other liquids of lighter
or denser specific gravity, as the case may be, their proper
classification may easily be arrived at, and if the results are checked
by actual weight, in a specific gravity balance, they will be found to
be fairly accurate. The solution commonly used for the heaviest stones
is a mixture of nitrate of thallium and nitrate of silver. This double
nitrate has a specific gravity of 4.7963, therefore such a stone as
zircon, which is the heaviest known, will float in it. For use, the
mixture should be slightly warmed till it runs thin and clear; this is
necessary, because at 60° (taking this as ordinary atmospheric
temperature) it is a stiff mass. A lighter liquid is a mixture of iodide
of mercury in iodide of potassium, but this is such an extremely
corrosive and dangerous mixture, that the more common solution is one
in which methylene iodide is saturated with a mixture of iodoform until
it shows a specific gravity of 3.601; and by using the methylene iodide
alone, in its pure state, it having a specific gravity of 3.2981, the
stones to that weight can be isolated, and by diluting this with
benzole, its weight can be brought down to that of the benzole itself,
as in the case of Sonstadt's solution. This solution, in full standard
strength, has a specific gravity of 3.1789, but may be weakened by the
addition of distilled water in varying proportions till the weight
becomes almost that of water.
Knowing the specific gravity of all stones, and dividing them into six
groups, by taking a series of standard solutions selected from one or
other of the above, and of known specific gravity, we can judge with
accuracy if any stone is what it is supposed to be, and classify it
correctly by its mere floating or sinking when placed in these liquids.
Beginning then with the pure double nitrate of silver and thallium, this
will isolate the stones of less specific gravity than 4.7963, and taking
the lighter solutions and standardising them, we may get seven solutions
which will isolate the stones as follows:--
A {shows the stones which have} 4.7963
{a specific gravity over}
B " " " 3.70 and under 4.7963
C " " " 3.50 " 3.70
D " " " 3.00 " 3.50
E " " " 2.50 " 3.00
F " " " 2.00 " 2.50
G " " -- -- under 2.00
Therefore each liquid will isolate the stones in its own group by
compelling them to float on its surface; commencing with the heaviest
and giving to the groups the same letters as the liquids, it is seen
that--
_Group_ A.--Isolates gems with a specific gravity of 4.7963 and over
4.70; in this group is placed zircon, with a specific gravity of from
4.70 to 4.88.
_Group_ B.--Stones whose specific gravity lies between 3.70 and under
4.7963.
Garnets, many varieties. See Group D below.
Almandine 4.11 and occasionally to 4.25
Ruby 4.073 " 4.080
Sapphire 4.049 " 4.060
Corundum 3.90 " 4.16
Cape Ruby 3.861
Demantoid 3.815
Staurolite 3.735
Malachite 3.710 and occasionally to 3.996
_Group_ C.--Stones whose specific gravity lies between 3.50 and under
3.70.
Pyrope (average) 3.682
Chrysoberyl 3.689 and occasionally to 3.752
Spinel 3.614 " 3.654
Kyanite 3.609 " 3.688
Hessonite 3.603 " 3.651
Diamond 3.502 " 3.564
Topaz 3.500 " 3.520
_Group_ D.--Stones whose specific gravity lies between 3 and under 3.50.
Rhodonite 3.413 and occasionally to 3.617
Garnets 3.400 " 4.500
Epidote 3.360 " 3.480
Sphene 3.348 and occasionally to 3.420
Idocrase 3.346 " 3.410
Olivine 3.334 " 3.368
Chrysolite 3.316 " 3.528
Jade 3.300 " 3.381
Jadeite 3.299
Axinite 3.295
Dioptase 3.289
Diopside 2.279
Tourmaline (yellow) 3.210
Andalusite 3.204
Apatite 3.190
Tourmaline (Blue and
Violet) 3.160
Tourmaline (Green) 3.148
" (Red) 3.100
Spodumene 3.130 and occasionally to 3.200
Euclase 3.090
Fluorspar 3.031 and occasionally to 3.200
Tourmaline (Colourless) 3.029
Tourmaline (Blush
Rose) 3.024
Tourmaline (Black) 3.024 and occasionally to 3.300
Nephrite 3.019
_Group_ E.--Stones whose specific gravity lies between 2.50 and under
3.000.
Phenakite 2.965
Turquoise 2.800
Beryl 2.709 and occasionally to 2.81
Aquamarine 2.701 " 2.80
Labradorite 2.700
Emerald 2.690
Quartz 2.670
Chrysoprase 2.670
Jasper 2.668
Amethyst 2.661
Hornstone 2.658
Citrine 2.658
Cordierite 2.641
Agate 2.610
Chalcedony 2.598 and occasionally to 2.610
Adularia 2.567
Rock-crystal 2.521 and occasionally to 2.795
_Group_ F.--Stones whose specific gravity lies between 2.00 and under
2.50.
Haüynite 2.470 and occasionally to 2.491
Lapis lazuli 2.461
Moldavite 2.354
Opal 2.160 and according to variety to 2.283
" (Fire Opal) 2.210 (average)
_Group_ G.--Stones whose specific gravity is under 2.00.
Jet 1.348
Amber 1.000
(See also list of stones, arranged in their respective colours,
in Chapter XII.)
In many of these cases the specific gravity varies from .11 to .20, but
the above are the average figures obtained from a number of samples
specially and separately weighed. In some instances this difference may
cause a slight overlapping of the groups, as in group C, where the
chrysoberyl may weigh from 3.689 to 3.752, thus bringing the heavier
varieties of the stone into group B, but in all cases where overlapping
occurs, the colour, form, and the self-evident character of the stone
are in themselves sufficient for classification, the specific gravity
proving genuineness. This is especially appreciated when it is
remembered that so far science has been unable (except in very rare
instances of no importance) to manufacture any stone of the same colour
as the genuine and at the same time of the same specific gravity. Either
the colour and characteristics suffer in obtaining the required weight
or density, or if the colour and other properties of an artificial stone
are made closely to resemble the real, then the specific gravity is so
greatly different, either more or less, as at once to stamp the jewel as
false. In the very few exceptions where chemically-made gems even
approach the real in hardness, colour, specific gravity, &c., they cost
so much to obtain and the difficulties of production are so great that
they become mere chemical curiosities, far more costly than the real
gems. Further, they are so much subject to chemical action, and are so
susceptible to their surroundings, that their purity and stability
cannot be maintained for long even if kept airtight; consequently these
ultra-perfect "imitations" are of no commercial value whatever as
jewels, even though they may successfully withstand two or three tests.
CHAPTER IX.
PHYSICAL PROPERTIES.
G--HEAT.
Another method of isolating certain stones is by the action of
heat-rays. Remembering our lessons in physics we recall that just as
light-rays may be refracted, absorbed, or reflected, according to the
media through which they are caused to pass, so do heat-rays possess
similar properties. Therefore, if heat-rays are projected through
precious stones, or brought to bear on them in some other manner than by
simple projection, they will be refracted, absorbed, or reflected by the
stones in the same manner as if they were light-rays, and just as
certain stones allow light to pass through their substance, whilst
others are opaque, so do some stones offer no resistance to the passage
of heat-rays, but allow them free movement through the substance,
whilst, in other cases, no passage of heat is possible, the stones being
as opaque to heat as to light. Indeed, the properties of light and heat
are in many ways identical, though the test by heat must in all cases
give place to that by light, which latter is by far of the greater
importance in the judging and isolation of precious stones. It will
readily be understood that in the spectrum the outer or extreme
light-rays at each side are more or less bent or diverted, but those
nearest the centre are comparatively straight, so that, as before
remarked, these central rays are taken as being the standard of
light-value. This divergence or refraction is greater in some stones
than in others, and to it the diamond, as an example, owes its chief
charm. In just such manner do certain stones refract, absorb, or reflect
heat; thus amber, gypsum, and the like, are practically opaque to
heat-rays, in contrast with those of the nature of fluorspar, rock-salt,
&c., which are receptive. Heat passes through these as easily as does
light through a diamond, such stones being classed as diathermal (to
heat through). So that all diathermal stones are easily permeable by
radiant heat, which passes through them exactly as does light through
transparent bodies.
Others, again, are both single and double refracting to heat-rays, and
it is interesting to note the heat-penetrating value as compared with
the refractive indexes of the stone. In the following table will be
found the refractive indexes of a selection of single and double
refractive stones, the figures for "Light" being taken from a standard
list. The second column shows the refractive power of heat, applied to
the actual stones, and consisting of a fine pencil blowpipe-flame, one
line (the one twelfth part of an inch) in length in each case. This list
must be taken as approximate, since in many instances the test has been
made on one stone only, without possibility of obtaining an average; and
as stones vary considerably, the figures may be raised or lowered
slightly, or perhaps even changed in class, because in some stones the
least stain or impurity may cause the heat effects to be altered greatly
in their character, and even to become singly or doubly refracting,
opaque or transparent, to heat-rays, according to the nature of the
impurity or to some slight change in the crystalline structure, and so
on.
_Selection of Singly refracting stones._ _Indexes of Rays of_
LIGHT. HEAT.
Fluorspar 1.436 4.10 varies
Opal 1.479 2.10 "
Spinel 1.726 1.00
Almandine 1.764 1.00
Diamond 2.431 6.11 double
_Selection of Doubly refracting stones._ _Indexes of Rays of_
LIGHT. HEAT.
Quartz 1.545 4.7 single and double
Beryl 1.575 1.0 varies considerably
Topaz 1.635 4.1 " "
Chrysoberyl 1.765 1.1 " "
Ruby 1.949 5.1 single and double
The tourmaline has a light-refractive index of 1.63, with a heat index
of none, being to heat-rays completely opaque.
The refractive index of gypsum is 1.54, but heat none, being opaque.
The refractive index of amber is 1.51, but heat none, being opaque.
In some of the specimens the gypsum showed a heat-penetration index of
0.001, and amber of 0.056, but mostly not within the third point. In all
cases the heat-penetration and refraction were shown by electric
recorders. These figures are the average of those obtained from tests
made in some cases on several stones of the same kind, and also on
isolated specimens. Not only does the power of the stone to conduct
heat vary in different stones of the same kind or variety, as already
explained, but there is seen a remarkable difference in value, according
to the spot on which the heat is applied, so that on one stone there is
often seen a conductivity varying between 0.15 to 4.70.
This is owing to the differences of expansion due to the temporary
disturbance of its crystalline structure, brought about by the applied
heat. This will be evident when heat is applied on the axes of the
crystal, on their faces, angles, lines of symmetry, etc., etc., each one
of which gives different results, not only as to value in conductivity,
but a result which varies in a curious degree, out of all proportion to
the heat applied. In many cases a slight diminution in applied heat
gives a greater conductivity, whilst in others a slight rise in the
temperature of the heat destroys its conductivity altogether, and
renders the stone quite opaque to heat-rays.
This anomaly is due entirely to the alteration of crystalline structure,
which, in the one case, is so changed by the diminution in heat as to
cause the crystals to be so placed that they become diathermal, or
transparent to heat-rays; whilst, in the other instance, the crystals
which so arrange themselves as to be diathermal are, by a slightly
increased temperature, somewhat displaced, and reflect, or otherwise
oppose the direct passage of heat-rays, which, at the lower temperature,
obtained free passage.
Thus certain stones become both opaque and diathermal, and as the heat
is caused to vary, so do they show the complete gamut between the two
extremes of total opacity and complete transparency to heat-rays.
For the purpose under consideration, the temperature of the pencil of
heat applied to the stones in their several portions was kept constant.
It will be seen, therefore, that no great reliance can be placed on the
heat test as applied to precious stones.
CHAPTER X.
PHYSICAL PROPERTIES.
H--MAGNETIC AND ELECTRIC INFLUENCES.
The word "electricity" is derived from the Greek "elektron," which was
the name for amber, a mineralised resin of extinct pine-trees. It was
well-known to the people of pre-historic times; later to the early
Egyptians, and, at a still later date, we have recorded how Thales--the
Greek philosopher, who lived about the close of the 7th Century B.C.,
and was one of the "seven wise men"--discovered the peculiar property
which we call "electricity" by rubbing dry silk on amber.
Many stones are capable of exhibiting the same phenomenon, not only by
friction, as in Thales's experiment, but also under the influence of
light, heat, magnetism, chemical action, pressure, etc., and of holding
or retaining this induced or added power for a long or short period,
according to conditions and environment.
If a small pith ball is suspended from a non-conducting support, it
forms a simple and ready means of testing the electricity in a stone.
According to whether the ball is repelled or attracted, so is the
electricity in the stone made evident, though the electroscope gives the
better results. By either of these methods it will be found that some of
the stones are more capable of giving and receiving charges of
electricity than are others; also that some are charged throughout with
one kind only, either positive or negative, whilst others have both,
becoming polarised electrically, having one portion of their substance
negative, the other positive. For instance, amber, as is well known,
produces negative electricity under the influence of friction, but in
almost all cut stones, other than amber, the electricity produced by the
same means is positive, whereas in the _uncut_ stones the electricity is
negative, with the exception of the diamond, in which the electricity is
positive.
When heated, some stones lose their electricity; others develop it,
others have it reversed, the positive becoming negative and vice versâ;
others again, when heated, become powerfully magnetic and assume strong
polarity. When electricity develops under the influence of heat, or is
in any way connected with a rising or falling of temperature in a body,
it is called "pyro-electricity," from the Greek word "pyros," fire. The
phenomenon was first discovered in the tourmaline, and it is observed,
speaking broadly, only in those minerals which are hemimorphic, that is,
where the crystals have different planes or faces at their two ends,
examples of which are seen in such crystals as those of axinite,
boracite, smithsonite, topaz, etc., all of which are hemimorphic.
Taking the tourmaline as an example of the pyro-electric minerals, we
find that when this is heated to between 50° F. and 300° F. it assumes
electric polarity, becoming electrified positively at one end or pole
and negatively at the opposite pole. If it is suspended on a silken
thread from a glass rod or other non-conducting support in a similar
manner to the pith ball, the tourmaline will be found to have become an
excellent magnet. By testing this continually as it cools there will
soon be perceived a point which is of extreme delicacy of temperature,
where the magnetic properties are almost in abeyance. But as the
tourmaline cools yet further, though but a fraction of a degree, the
magnetic properties change; the positive pole becomes the negative, the
negative having changed to the positive.
It is also interesting to note that if the tourmaline is not warmed so
high as to reach a temperature of 50° F., or is heated so strongly as to
exceed more than a few degrees above 300° F., then these magnetic
properties do not appear, as no polarity is present. This polarity, or
the presence of positive and negative electricity in one stone, may be
strikingly illustrated in a very simple manner:--If a little sulphur and
red-lead, both in fine powder, are shaken up together in a paper or
similar bag, the moderate friction of particle against particle
electrifies both; one negatively, the other positively. If, then, a
little of this now golden-coloured mixture is gently dusted over the
surface of the tourmaline or other stone possessing electric polarity, a
most interesting change is at once apparent. The red-lead separates
itself from the sulphur and adheres to the negative portion of the
stone, whilst the separated sulphur is at once attracted to the positive
end, so that the golden-coloured mixture becomes slowly transformed into
its two separate components--the brilliant yellow sulphur, and the
equally brilliant red-lead. These particles form in lines and waves
around the respective poles in beautiful symmetry, their positions
corresponding with the directions of the lines of magnetic force,
exactly as will iron filings round the two poles of a magnet.
From this it will clearly be seen how simple a matter it is to isolate
the topaz, tourmaline, and all the pyro-electric stones from the
non-pyro-electric, for science has not as yet been able to give to
spurious stones these same electric properties, however excellent some
imitations may be in other respects. Further, almost all minerals lose
their electricity rapidly on exposure to atmospheric influences, even to
dry air; the diamond retains it somewhat longer than most stones, though
the sapphire, topaz, and a few others retain it almost as long again as
the diamond, and these electric properties are some of the tests which
are used in the examination of precious stones.
Those stones which show electricity on the application of pressure are
such as the fluorspar, calcite, and topaz.
With regard to magnetism, the actual cause of this is not yet known with
certainty. It is, of course, a self-evident fact that the magnetic iron
ore, which is a form of peroxide, commonly known as magnetite, or
lodestone, has the power of attracting a magnet when swinging free, or
of being attracted by a magnet, to account for which many plausible
reasons have been advanced. Perhaps the most reasonable and acceptable
of these is that this material contains molecules which have half their
substance positively and the other half negatively magnetised.
Substances so composed, of which magnets are an example, may be made
the means of magnetising other substances by friction, without they
themselves suffering any loss; but it is not all substances that will
respond to the magnet. For instance, common iron pyrites, FeS_{2}, is
unresponsive, whilst the magnetic pyrites, which varies from 5FeS,
Fe_{2}S_{3}, to 6FeS, Fe_{2}S_{3}, and is a sulphide of iron, is
responsive both positively and negatively. Bismuth and antimony also are
inactive, whilst almost all minerals containing even a small percentage
of iron will deflect the magnetic needle, at least under the influence
of heat. So that from the lodestone--the most powerfully magnetic
mineral known--to those minerals possessing no magnetic action whatever,
we have a long, graduated scale, in which many of the precious stones
appear, those containing iron in their composition being more or less
responsive, as already mentioned, and that either in their normal state,
or when heated, and always to an extent depending on the quantity or
percentage of iron they contain.
In this case, also, science has not as yet been able to introduce into
an artificial stone the requisite quantity of iron to bring it the same
analytically as the gem it is supposed to represent, without completely
spoiling the colour. So that the behaviour of a stone in the presence of
a magnet, to the degree to which it should or should not respond, is one
of the important tests of a genuine stone.
CHAPTER XI.
THE CUTTING OF PRECIOUS STONES.
As existing in a state of nature precious stones do not, as a rule,
exhibit any of those beautiful and wonderful properties which cause them
to be so admired and sought after as to become of great intrinsic value,
for their surfaces have become clouded by innumerable fine cuts or
abrasions, because of the thousands of years during which they have been
under pressure, or tumbled about in rivers, or subjected to the
incessant friction caused by surrounding substances. All this occurring
above and under ground has given them an appearance altogether different
to that which follows cutting and polishing. Further, the shape of the
stone becomes altered by the same means, and just as Michael Angelo's
figure was already in the marble, as he facetiously said, and all he had
to do was to chip off what he did not require till he came to it, so is
the same process of cutting and polishing necessary to give to the
precious stones their full value, and it is the manner in which these
delicate and difficult operations are performed that is now under
consideration. Just as experience and skill are essential to the
obtaining of a perfect figure from the block of marble, so must the
cutting and polishing of a precious stone call for the greatest
dexterity of which a workman is capable, experience and skill so great
as to be found only in the expert, for in stones of great value even a
slight mistake in the shaping and cutting would probably not only be
wasteful of the precious material, but would utterly spoil its beauty,
causing incalculable loss, and destroying altogether the refrangibility,
lustre and colour of the stone, thus rendering it liable to easy
fracture: in every sense converting what would have been a rare and
magnificent jewel to a comparatively valueless specimen.
One of the chief services rendered by precious stones is that they may
be employed as objects of adornment, therefore, the stone must be cut of
such a shape as will allow of its being set without falling out of its
fastening--not too shallow or thin, to make it unserviceable and liable
to fracture, and in the case of a transparent stone, not too deep for
the light to penetrate, or much colour and beauty will be lost. Again,
very few stones are flawless, and the position in which the flaw or
flaws appear will, to a great extent, regulate the shape of the stones,
for there are some positions in which a slight flaw would be of small
detriment, because they would take little or no reflection, whilst in
others, where the reflections go back and forth from facet to facet
throughout the stone, a flaw would be magnified times without number,
and the value of the stone greatly reduced. It is therefore essential
that a flaw should be removed whenever possible, but, when this is not
practicable, the expert will cut the stone into such a shape as will
bring the defect into the least important part of the finished gem, or
probably sacrifice the size and weight of the original stone by cutting
it in two or more pieces of such a shape that the cutting and polishing
will obliterate the defective portions. Such a method was adopted with
the great Cullinan diamond, as described in Chapter IV. From this
remarkable diamond a great number of magnificent stones were obtained,
the two chief being the largest and heaviest at present known. Some idea
of the size of the original stone may be gathered from the fact that the
traditional Indian diamond, the "Great Mogul," is said to have weighed
280 carats. This stone, however, is lost, and some experts believe that
it was divided, part of it forming the present famous Koh-i-nûr; at any
rate, all trace of the Great Mogul ceased with the looting of Delhi in
1739. The Koh-i-nûr weighs a little over 106 carats; before cutting it
weighed a shade over 186; the Cullinan, in the same state, weighed
nearly 3254 carats. This massive diamond was cut into about 200 stones,
the largest, now placed in "The Royal Sceptre with the Cross," weighing
516-1/2 carats, the second, now placed under the historic ruby in "The
Imperial State Crown," weighing 309-3/16ths carats. These two diamonds
are now called "The Stars of Africa." Both these stones, but especially
the larger, completely overshadow the notorious Koh-i-nûr, and
notwithstanding the flaw which appeared in the original stone, every one
of the resulting pieces, irrespective of weight, is without the
slightest blemish and of the finest colour ever known, for the great
South African diamond is of a quality never even approached by any
existing stone, being ideally perfect.
It requires a somewhat elaborate explanation to make clear the various
styles of cut without illustrations. They are usually divided into two
groups, with curved, and with flat or plane surfaces. Of the first, the
curved surfaces, opaque and translucent stones, such as the moonstone,
cat's-eye, etc., are mostly cut _en cabochon_, that is, dome-shaped or
semi-circular at the top, flat on the underside, and when the garnet is
so cut it is called a carbuncle. In strongly coloured stones, while the
upper surface is semi-circular like the cabochon, the under surface is
more or less deeply concave, sometimes following the curve of the upper
surface, the thickness of the stone being in that case almost parallel
throughout. This is called the "hollow" cabochon. Other stones are cut
so that the upper surface is dome-shaped like the last two, but the
lower is more or less convex, though not so deep as to make the stone
spherical. This is called the "double" cabochon.
A further variety of cutting is known as the _goutte de suif_, or the
"tallow-drop," which takes the form of a somewhat flattened or
long-focus double-convex lens. The more complicated varieties of cut are
those appearing in the second group, or those with plane surfaces. A
very old form is the "rose" or "rosette"; in this the extreme upper
centre, called the "crown," or "star," is usually composed of six
triangles, the apexes of which are elevated and joined together, forming
one point in the centre. From their bases descend a further series of
triangles, the bases and apexes of which are formed by the bases and
lower angles of the upper series. This lower belt is called the "teeth,"
under which the surface or base of the stone is usually flat, but
sometimes partakes of a similar shape to the upper surface, though
somewhat modified in form.
Another variety is called the "table cut," and is used for coloured
stones. It has a flat top or "table" of a square or other shape, the
edges of which slope outwards and form the "bezils" or that extended
portion by which the stone is held in its setting. It will thus be seen
that the outside of the stone is of the same shape as that of the
"table," but larger, so that from every portion of the "table" the
surface extends downwards, sloping outwards to the extreme size of the
stone, the underside sloping downwards and inwards to a small and flat
base, the whole, in section, being not unlike the section of a "pegtop."
A modification of this is known as the "step" cut, sometimes also called
the "trap." Briefly, the difference between this and the last is that
whereas the table has usually one bevel on the upper and lower surfaces,
the trap has one or more steps in the sloping parts, hence its name.
The most common of all, and usually applied only to the diamond, is the
"brilliant" cut. This is somewhat complicated, and requires detailed
description. In section, the shape is substantially that of a pegtop
with a flat "table" top and a small flat base. The widest portion is
that on which the claws, or other form of setting, hold it securely in
position. This portion is called the "girdle," and if we take this as a
defining line, that portion which appears above the setting of this
girdle, is called the "crown"; the portion below the girdle is called
the "culasse," or less commonly the "pavilion." Commencing with the
girdle upwards, we have eight "cross facets" in four pairs, a pair on
each side; each pair having their apexes together, meeting on the four
extremities of two lines drawn laterally at right angles through the
stone. It will, therefore, be seen that one side of each triangle
coincides with the girdle, and as their bases do not meet, these spaces
are occupied by eight small triangles, called "skill facets," each of
which has, as its base, the girdle, and the outer of its sides coincides
with the base of the adjoining "cross facet." The two inner sides of
each pair of skill facets form the half of a diamond or lozenge-shaped
facet, called a "quoin," of which there are four. The inner or upper
half of each of these four quoins forms the bases of two triangles, one
at each side, making eight in all, which are called "star facets," and
the inner lines of these eight star facets form the boundary of the top
of the stone, called the "table." The inner lines also of the star
facets immediately below the table and those of the cross facets
immediately above the girdle form four "templets," or "bezils." We thus
have above the girdle, thirty-three facets: 8 cross, 8 skill, 4 quoin, 8
star, 1 table, and 4 templets.
Reversing the stone and again commencing at the girdle, we have eight
"skill facets," sometimes called the lower skill facets, the bases of
which are on the girdle, their outer sides forming the bases of eight
cross facets, the apexes of which meet on the extremities of the
horizontal line, as in those above the girdle. If the basal lines of
these cross facets, where they join the sides of the skill facets, are
extended to the peak, or narrow end of the stone, these lines, together
with the sides of the cross facets, will form four five-sided facets,
called the "pavilions"; the spaces between these four pavilions have
their ends nearest the girdle formed by the inner sides of the skill
facets, and of these spaces, there will, of course, be four, which also
are five-sided figures, and are called "quoins," so that there are eight
five-sided facets--four large and four narrow--their bases forming a
square, with a small portion of each corner cut away; the bases of the
broader pavilions form the four sides, whilst the bases of the four
narrower quoins cut off the corners of the square, and this flat
portion, bounded by the eight bases, is called the "culet," but more
commonly "collet." So that below the girdle, we find twenty-five facets:
8 cross, 8 skill, 4 pavilion, 4 quoin, and 1 collet.
These, with the 33 of the crown, make 58, which is the usual number of
facets in a brilliant, though this varies with the character, quality,
and size of the diamond. For instance, though this number is considered
the best for normal stones, specially large ones often have more,
otherwise there is danger of their appearing dull, and it requires a
vast amount of skill and experience to decide upon the particular number
and size of the facets that will best display the fire and brilliance of
a large stone, for it is obvious that if, after months of cutting and
polishing, it is found that a greater or smaller number of facets ought
to have been allowed, the error cannot be retrieved without considerable
loss, and probable ruin to the stone. In the case of the Cullinan
diamonds, the two largest of which are called the Stars of Africa, 74
facets were cut in the largest portion, while in the next largest the
experts decided to make 66, and, as already pointed out, these stones
are, up to the present time, the most magnificent in fire, beauty and
purity ever discovered.
The positions and angles of the facets, as well as the number, are of
supreme importance, and diamond cutters--even though they have rules
regulating these matters, according to the weight and size of the
stone--must exercise the greatest care and exactitude, for their
decision once made is practically unalterable.
CHAPTER XII.
IMITATIONS, AND SOME OF THE TESTS, OF PRECIOUS STONES.
We now arrive at the point where it is necessary to discuss the
manufacture and re-formation of precious stones, and also to consider a
few of the tests which may be applied to _all_ stones. These are given
here in order to save needless repetition; the tests which are specially
applicable to individual stones will more properly be found under the
description of the stone referred to, so that the present chapter will
be devoted chiefly to generalities.
With regard to diamonds, the manufacture of these has not as yet been
very successful. As will be seen on reference to Chapter II., on "the
Origin of Precious Stones," it is generally admitted that these
beautiful and valuable minerals are caused by chemically-charged water
and occasionally, though not always, high temperature, but invariably
beautified and brought to the condition in which they are obtained by
the action of weight and pressure, extending unbroken through perhaps
ages of time.
In these circumstances, science, though able to give chemical
properties and pressure, cannot, of course, maintain these continuously
for "ages," therefore the chemist must manufacture the jewels in such
manner that he may soon see the results of his labours, and though real
diamonds may be made, and with comparative ease, from boron in the
amorphous or pure state along with aluminium, fused in a crucible at a
high temperature, these diamonds are but microscopic, nor can a number
of them be fused, or in any other way converted into a large single
stone, so that imitation stones, to be of any service must be made of a
good clear glass. The glass for this purpose is usually composed of
53.70 per cent. of red lead, 38.48 per cent. of pure quartz in fine
powder, preferably water-ground, and 7.82 per cent. of carbonate of
potash, the whole coloured when necessary with metallic oxides of a
similar nature to the constituents of the natural stones imitated. But
for colourless diamonds, the glass requires no such addition to tint it.
From the formula given is made the material known as "strass," or
"paste," and stones made of it are mostly exhibited under and amongst
brilliant artificial lights. The mere fact that they are sold cheaply is
_primâ facie_ proof that the stones are glass, for it is evident that a
diamond, the commercial value of which might be £50 or more, cannot be
purchased for a few shillings and be genuine. So long as this is
understood and the stone is sold for the few shillings, no harm is done;
but to offer it as a genuine stone and at the price of a genuine stone,
would amount to fraud, and be punishable accordingly. Some of these
"paste," or "white stones," as they are called in the trade, are cut and
polished exactly like a diamond, and with such success as occasionally
to deceive all but experts. Such imitations are costly, though, of
course, not approaching the value of the real stones; it being no
uncommon thing for valuable jewels to be duplicated in paste, whilst the
originals are kept in the strong room of a bank or safe-deposit.
In all cases, however, a hard file will abrade the surface of the false
stone. In chapter VII. we found that quartz is in the seventh degree of
hardness, and an ordinary file is but a shade harder than this, so that
almost all stones higher than No. 7 are unaffected by a file unless it
is used roughly, so as to break a sharp edge. In order to prepare
artificial diamonds and other stones for the file and various tests,
they are often what is called "converted" into "doublets" or "triplets."
These are made as follows: the body of the glass is of paste, and on the
"table" (see last chapter), and perhaps on the broader facets, there
will be placed a very thin slab of the real stone, attached by cement.
In the case of the diamond, the body is clear, but in the coloured
imitations the paste portion is made somewhat lighter in shade than the
real stone would be, the portion below the girdle being coloured
chemically, or mounted in a coloured backing. Such a stone will, of
course, stand most tests, for the parts usually tested are genuine.
A stone of this nature is called a "doublet," and it is evident that
when it is tested on the underside, it will prove too soft, therefore
the "triplet" has been introduced. This is exactly on the lines of the
doublet, except that the collet and perhaps the pavilions are covered
also, so that the girdle, which is generally encased by the mounting,
is the only surface-portion of paste. In other cases the whole of the
crown is genuine, whilst often both the upper and lower portions are
solid and genuine, the saving being effected by using a paste centre at
the girdle, covered by the mounting. Such a stone as this last mentioned
is often difficult to detect without using severe tests and desperate
means, e.g.:--(a) by its crystalline structure (see Chapter III.);
(b) by the cleavage planes (see Chapter IV.); (c) by the polariscope
(see Chapter V.); (d) by the dichroscope (see Chapter VI.); (e) by
specific gravity (see Chapter VIII.); (f) cutting off the mounting,
and examining the girdle; (g) soaking the stone for a minute or so in
a mixture said to have been originally discovered by M. D. Rothschild,
and composed of hydrofluoric acid and ammonia; this will not answer for
all stones, but is safe to use for the diamond and a few others. Should
the jewel be glass, it will be etched, if not completely destroyed, but
if genuine, no change will be apparent; (h) soaking the diamond for a
few minutes in warm or cold water, in alcohol, in chloroform, or in all
these in turn, when, if a doublet, or triplet, it will tumble to pieces
where joined together by the cement, which will have been dissolved. It
is, however, seldom necessary to test so far, for an examination under
the microscope, even with low power, is usually sufficient to detect in
the glass the air-bubbles which are almost inseparable from
glass-mixtures, though they do not detract from the physical properties
of the glass. The higher powers of the same instrument will almost
always define the junction and the layer or layers of cement, no matter
how delicate a film may have been used. Any one of these tests is
sufficient to isolate a false stone.
Some of the softer genuine stones may be fused together with splinters,
dust, and cuttings of the same stones, and of this product is formed a
larger stone, which, though manufactured, is essentially perfectly real,
possessing exactly the same properties as a naturally formed stone. Many
such stones are obtained as large as an ordinary pin's head, and are
much used commercially for cluster-work in rings, brooches, for
watch-jewels, scarf-pins, and the like, and are capable of being cut and
polished exactly like an original stone. This is a means of using up to
great advantage the lapidary's dust, and though these products are real
stones, perhaps a little more enriched in colour chemically, they are
much cheaper than a natural stone of the same size and weight.
Some spurious stones have their colour improved by heat, by being tinged
on the outside, by being tinted throughout with a fixed colour and
placed in a clear setting; others, again, have a setting of a different
hue, so that the reflection of this shall give additional colour and
fire to the stone. For instance, glass diamonds are often set with the
whole of the portion below the girdle hidden, this part of the stone
being silvered like a mirror. Others are set open, being held at the
girdle only, the portion covered by the setting being silvered. Other
glass imitations, such as the opal, have a tolerably good representation
of the "fiery" opal given to them by the admixture, in the glass, of a
little oxide of tin, which makes it somewhat opalescent, and in the
setting is placed a backing of red, gold, copper, or fiery-coloured
tinsel, whilst the glass itself, at the back, is painted very thinly
with a paint composed of well washed and dried fish-scales, reduced to
an impalpable powder, mixed with a little pure, refined mastic, or other
colourless varnish. This gives a good imitation of phosphorescence, as
well as a slight pearliness, whilst the tinsel, seen through the paint
and the curious milkiness of the glass, gives good "fire."
A knowledge of the colours natural to precious stones and to jewels
generally is of great service in their rough classification for testing,
even though some stones are found in a variety of colours. An
alphabetical list of the most useful is here appended, together with
their average specific gravities and hardness. (See also Chapter VII. on
"Hardness," and Chapter VIII. on "Specific Gravity.")
WHITE OR COLOURLESS STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Beryl 7-3/4 2.709-2.81
Corundum 9 3.90-4.16
Diamond 10 3.502-3.564
Jade 7 3.300-3.381
Opal 5-1/2-6-1/2 2.160-2.283
Phenakite 7-3/4 2.965
Quartz 7 2.670
Rock-crystal 7 2.521-2.795
Sapphire 9 4.049-4.060
Spinel 8 3.614-3.654
Topaz 8 3.500-3.520
Tourmaline 7-1/4 3.029
Zircon 7-1/2 4.700-4.880
YELLOW STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Amber 2-1/2 1.000
Beryl 7-3/4 2.709-2.810
Chrysoberyl 8-1/2 3.689-3.752
Chrysolite 6-7 3.316-3.528
Corundum (the yellow variety known
as "Oriental Topaz" [not "Topaz"],
see below) 9 3.90-4.16
Diamond 10 3.502-3.564
Garnets (various) 6-1/2-7-1/2 3.4-4.5
Hyacinth (a form of Zircon) 7-1/2 4.7-4.88
Quartz (Citrine) 7 2.658
Sapphire 9 4.049-4.060
Spinel 8 3.614-3.654
Topaz (for "Oriental Topaz," see above) 8 3.500-3.520
Tourmaline 7-1/4 3.210
BROWN AND FLAME-COLOURED STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Andalusite 7-1/2 3.204
Diamond 10 3.502-3.564
Garnets (various) 6-1/2-7-1/2 3.40-4.50
Hyacinth (a form of Zircon), see below 7-1/2 4.70-4.88
Quartz (smoke coloured) 7 2.670
Tourmaline 7-1/4 3.100
Zircon (Hyacinth) 7-1/2 4.70-4.88
RED AND ROSE-COLOURED STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Carnelian (a variety of Chalcedony) 6-1/2 2.598-2.610
Diamond 10 3.502-3.564
Deep Red Garnet 7-1/4 3.40-4.50
Jasper 7 2.668
Opal (the "Fire Opal") 5-1/2-6-1/2 2.21
(average)
Ruby 9 4.073-4.080
Rhodonite 5-1/2-6-1/2 3.413-3.617
Sapphire 9 4.049-4.060
Spinel Ruby 8 3.614-3.654
Topaz 8 3.500-3.520
Tourmaline 7-1/4 3.024
Zircon 7-1/2 4.70-4.88
PINK STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Beryl 7-3/4 2.709-2.810
Diamond 10 3.502-3.564
Ruby 9 4.073-4.080
Spinel 8 3.614-3.654
Topaz ("burnt" or "pinked"), see
Chapter XIV., page 92 8 3.500-3.520
Tourmaline 7-1/4 3.024
BLUE STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Beryl 7-3/4 2.709-2.810
Diamond 10 3.502-3.564
Dichorite (Water Sapphire) 7-7-1/2 4.049-4.060
Disthene (Kyanite) 5-7 3.609-3.688
Iolite (Cordierite) 7-1/4 2.641
Lapis lazuli 5-1/2 2.461
Sapphire 9 4.049-4.060
Topaz 8 3.500-3.520
Tourmaline 7-1/4 3.160
Turquoise 6 2.800
GREEN STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Aquamarine 7-3/4 2.701-2.800
Chrysoberyl 8-1/2 3.689-3.752
Chrysolite 6-7 3.316-3.528
Chrysoprase (Quartz) 7 2.670
Diamond 10 3.502-3.564
Dioptase 5 3.289
Emerald and Oriental Emerald 7-3/4 2.690
Euclase 7-1/2 3.090
Garnet (see also Red Garnet) 6-1/2-7-1/2 3.400-4.500
Heliotrope (Chalcedony) 6-1/2 2.598-2.610
Hiddenite (a variety of Spodumene) 6-1/2-7 3.130-3.200
Jade 7 3.300-3.381
Jadeite 7 3.299
Malachite 3-1/2 3.710-3.996
Peridot (a variety of Chrysolite) 6-7 3.316-3.528
Plasma (a variety of Chalcedony) 6-1/2 2.598-2.610
Quartz 7 2.670
Sapphire 9 4.049-4.060
Topaz 8 3.500-3.520
Tourmaline 7-1/4 3.148
VIOLET STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Amethyst 7 2.661
Diamond 10 3.502-3.564
Quartz (Amethyst) 7 2.670
Sapphire 9 4.049-4.060
Spinel 8 3.614-3.654
Tourmaline 7-1/4 3.160
CHATOYANT STONES.
These stones are easily recognisable by their play of colour. (See
Chapter XIV.)
BLACK STONES.
_Hardness._ _Specific Gravity._
(See Chapter VII.) (See Chapter VIII.)
Diamond 10 3.502-3.564
Garnet 6-1/2-7-1/2 3.400-4.500
Jet 3-1/2 1.348
Onyx (a variety of Chalcedony) 6-1/2 2.598-2.610
Quartz 7 2.670
Tourmaline (not unlike Black Resin
in appearance) 7-1/4 3.024-3.300
CHAPTER XIII.
VARIOUS PRECIOUS STONES.
_The Diamond._
To recapitulate certain of the facts respecting the diamond.--This
wonderful gem has the distinction amongst precious stones of being
unique; though many are composed of two, three, or but a small number of
elements, the diamond is the only stone known consisting of one element,
and absolutely nothing else--pure crystallised carbon. Its hardness is
proverbial; not only is it untouched by the action of a hard file, but
it occasionally refuses to split when struck with finely tempered steel,
which it often causes to break. Such was the case with the South African
diamond, for when the knife that was to break it was struck smartly with
a steel bar, the first blow broke the blade without affecting the
diamond, yet a piece of bort, or diamond dust, splinters, or defective
diamonds (all these being called bort), may readily be pulverised in a
hard steel mortar with a hard steel pestle.
The diamond is the hardest stone known; it is also the only stone known
which is really combustible. It is of true adamantine lustre, classed by
experts as midway between the truly metallic and the purely resinous. In
refractive power and dispersion of the coloured rays of light, called
its fire, it stands pre-eminent. It possesses a considerable variety of
colour; that regarded as the most perfect and rare is the blue-white
colour. Most commonly, however, the colours are clear, with steely-blue
casts, pale and neutral-colour yellow, whilst amongst the most expensive
and rare are those of green, pale pink, red, and any other variety with
strong and decided colour. Although these stones are sold by the carat,
there can be no hard and fast rule laid down as to the value of a carat,
for this depends on the size, quality, and the purity of the stone. The
larger the stone the greater the value per carat, and prices have been
known to range from 25_l._ per carat for a small stone to 500_l._ per
carat for a large one, whereas the exceptionally large stones possess a
value almost beyond estimation.
It often happens that some stones--particularly those from South Africa
and Brazil--are tinted when uncut, probably by reason of the action upon
them of their matrix, especially if ironstone, or with rolling for ages
amongst ironstone in river-beds, which gives them a slight metallic
appearance; in each case the cause is suggested by the fact that these
tinted stones are usually found in such places, and that the tinting is
very thin and on the surface only, so that the cutting and shaping of
the stone gets below it to the perfectly clear diamond.
From Pliny and other historians we gather that at various periods
considerable superstition has existed with regard to diamonds, such as
that if one is powdered it becomes poisonous to a remarkable degree;
that gifts of diamonds between lovers--married and unmarried--produce
and seal affection; hence the popularity of diamonds in betrothal
rings. Pretty as is this conceit, there is no doubt about the fact that
the gift of diamonds to the object of one's affections does usually
produce a feeling of pleasure to both parties, from which it would
appear that there is some ground for the belief.
_Corundum._
This mineral is a species of crystal, or crystalline alumina--an almost
pure anhydrous alumina, Al_{2}O_{3}--in many varieties, both of shape
and colour. The chief stone is the ruby, considered, when large, to be
of even more importance and value than the diamond. There are many other
red stones in this group; sapphires, also, are a species of corundum,
both the blue and the colourless varieties, as are also the aquamarine,
the emerald, the amethyst, the topaz, and others, all of widely
differing colour, as well as the star-shaped, or "aster" ruby, called
the "ruby" cat's-eye. All these vary more in colour than in their
chemical properties. Still another variety, greyish-black and generally
associated with hæmatite iron ore, is called emery, and, when ground in
different degrees of fineness, is so well known by its general use as a
polishing medium as to need no description. It should, however, be
mentioned that amongst the more coarsely ground emery it is no uncommon
thing to find minute sapphires, taking sapphires in their broad,
commercial meaning, as signifying any variety of corundum, except the
red and the emery. The surfaces of crystals of corundum are often
clouded or dull, whilst its classification of lustre is vitreous. It is
double refracting and has no cleavage. It is found in China, India,
Burma, Ceylon, South Africa, America, and in many other places, having a
wide distribution.
_The Ruby._
In the dichroscope the ruby shows two images, one square of a violet
red, the second square being a truer and a paler red. It may be
subjected to strong heat, when it changes its colour to a sooty or dirty
slate, this varying with the locality in which the stone is found, and
the manner in which the heat is applied. But as it cools it becomes
paler and greener, till it slowly enrichens; the green first becomes
broken, then warmer, redder, and finally assumes its original beautiful
blood red. This method of heating is sometimes used as a test, but it is
a test which often means the complete ruin of a stone which is not
genuine. Another characteristic which, in the eyes of the expert,
invariably isolates a real from an artificial ruby is its curious mild
brilliance, which as yet has not been reproduced by any scientific
method in paste or any other material, but perhaps the safest test of
all is the crystalline structure, which identical structure appears in
no other stone, though it is possible, by heating alumina coloured with
oxide of iron and perhaps also a trace of oxide of chromium to a very
high temperature for a considerable time, and then cooling very slowly,
to obtain a ruby which is nearly the same in its structure as the real
gem; its specific gravity and hardness may perhaps be to standard, and
when properly cut, its brilliance would deceive all but an expert. And
as in some real rubies there are found slight hollows corresponding or
analogous to the bubbles found in melted glass, it becomes a matter of
great difficulty to distinguish the real from the imitation by such
tests as hardness, specific gravity, dichroism, and the like, so that in
such a case, short of risking the ruin of the stone, ordinary persons
are unable to apply any convincing tests. Therefore, only the expert can
decide, by his appreciation of the delicate shade of difference in the
light of a true ruby and that of an excellent imitation, and by the
distribution of the colour, which--however experienced the chemist may
be, or with what care the colouring matter may have been incorporated in
the mass--has been found impossible of distribution throughout the body
of an artificial stone so perfectly and in the same manner and direction
as nature herself distributes it in the genuine. This alone, even in the
closest imitations, is clear to the eye of the expert, though not to the
untrained eye, unless the stone is palpably spurious. To one who is
accustomed to the examination of precious stones, however perfect the
imitation, it is but necessary to place it beside or amongst one or more
real ones for the false to be almost instantly identified, and that with
certainty.
_The Sapphire._
The Sapphire is not so easy to imitate, as its hardness exceeds that of
the ruby, and imitations containing its known constituents, or of glass,
are invariably softer than the natural stone. As before remarked, almost
any form of corundum other than red is, broadly, called sapphire, but
giving them their strictly correct designations, we have the olivine
corundum, called "chrysolite" (oriental), which is harder than the
ordinary or "noble" chrysolite, sometimes called the "peridot." The
various yellow varieties of corundum take the name of the "oriental
topaz," which, like most, if not all, the corundum varieties, is harder
than the gem which bears the same name, minus the prefix "oriental."
Then we have the "amethyst" sapphire, which varies from a red to a blue
purple, being richer in colour than the ordinary amethyst, which is a
form of violet-coloured quartz, but the corundum variety, which, like
its companions, is called the "oriental" amethyst, is both rarer and
more precious. A very rare and extremely beautiful green variety is
called the oriental emerald. The oriental jacinth, or hyacinth, is a
brown-red corundum, which is more stable than the ordinary hyacinth,
this latter being a form of zircon; it changes colour on exposure to
light, which colour is not restored by subsequent retention in darkness.
The blue sapphire is of all shades of blue, from cornflower blue to the
very palest tints of this colour, all the gradations from light to dark
purple blues, and, in fact, so many shades of tone and colour that they
become almost as numerous as the stones. These stones are usually found
in similar situations to those which produce the ruby, and often along
with them. The lighter colours are usually called females, or feminine
stones, whilst the darker ones are called masculine stones. Some of
these dark ones are so deep as to be almost black, when they are called
"ink" sapphires, and if inclining to blue, "indigo" sapphires, in
contradistinction to which the palest of the stones are called "water"
sapphires. The colouring matter is not always even, but is often spread
over the substance of the stone in scabs or "splotches," which rather
favours imitation, and, where this unevenness occurs, it may be
necessary to cut or divide the stone, or so to arrange the form of it
that the finished stone shall be equally blue throughout.
In some cases, however, the sapphire may owe its beauty to the presence
of two, three or more colours in separate strata appearing in one stone;
such as a portion being a green-blue, another a cornflower blue, another
perfectly colourless, another a pale sky blue, another yellow, each
perfectly distinct, the stone being cut so as to show each colour in its
full perfection.
This stone, the sapphire, is hardness No. 9 (see "Hardness" table), and
therefore ranks next to the diamond, which makes it a matter of great
difficulty to obtain an imitation which is of the same specific gravity
and of the same degree of hardness, though this has been done. Such
stones are purchasable, but though sold as imitations at comparatively
low price, and the buyer may consider them just as good as the real gem,
to the experienced eye they are readily detectable.
By heating a sapphire its blue colour slowly fades, to complete
transparency in many cases, or at any rate to so pale a tint as to pass
for a transparent stone. Valuable as is the sapphire, the diamond is
more so, and it follows that if one of these clear or "cleared"
sapphires is cut in the "rose" or "brilliant" form--which forms are
reserved almost exclusively for the diamond--such a stone would pass
very well as a diamond, and many so cut are sold by unscrupulous people
as the more valuable stone, which fraud an expert would, of course,
detect.
Sapphires are mentioned by Pliny, and figure largely in the ancient
history of China, Egypt, Rome, etc. The Greeks dedicated the sapphire
specially to Jupiter, and many of the stones were cut to represent the
god; it also figured as one of the chief stones worn by the Jewish High
Priest on the breast-plate. Some stones have curious rays of variegated
colour, due to their crystalline formation, taking the shape of a star;
these are called "asteriated," or "cat's eye" sapphires. Others have
curious flashes of light, technically called a "play" of light (as
described in Chapter VI. on "Colour"), together with a curious blue
opalescence; these are the "girasol." Another interesting variety of
this blue sapphire is one known as "chatoyant"; this has a rapidly
changing lustre, which seems to undulate between a green-yellow and a
luminous blue, with a phosphorescent glow, or fire, something like that
seen in the eyes of a cat in the dark, or the steady, burning glow
observed when the cat is fascinating a bird--hence its name. This is not
the same variety as the "asteriated," or "cat's eye" or "lynx eye"
mentioned above.
CHAPTER XIV.
VARIOUS PRECIOUS STONES--_continued._
_The Chrysoberyl._
There are certain stones and other minerals which, owing to their
possession of numerous microscopically fine cavities, of a globular or
tubular shape, have the appearance of "rays" or "stars," and these are
called "asteriated." Several of such stones have been discussed already
in the last chapter, and in addition to these star-like rays, some of
the stones have, running through their substance, one or more streaks,
perhaps of asbestos or calcite, some being perfectly clear, whilst
others are opalescent. When these streaks pass across the star-like
radiations they give the stone the appearance of an eye, the rays
forming the iris, the clear, opalescent, or black streak closely
resembling the slit in a cat's eye, and when these stones are cut _en
cabochon_, that is, dome-shaped (see Chapter XI. on "Cutting"), there is
nothing to deflect the light beams back and forth from facet to facet,
as in a diamond, so that the light, acting directly on these radiations
or masses of globular cavities and on the streak, causes the former to
glow like living fire, and the streak appears to vibrate, palpitate,
expand, and contract, exactly like the slit in the eye of a cat.
There are a considerable number of superstitions in connection with
these cat's-eye stones, many people regarding them as mascots, or with
disfavour, according to their colour. When possessing the favourite hue
or "fire" of the wearer, such as the fire of the opal for those born in
October, of the ruby for those born in July, etc., these stones are
considered to bring nothing but good luck; to ward off accident, danger,
and sudden death; to be a charm against being bitten by animals, and to
be a protection from poison, the "evil eye," etc. They figured largely,
along with other valuable jewels, in the worship of the ancient
Egyptians, and have been found in some of the tombs in Egypt. They also
appeared on the "systrum," which was a sacred instrument used by the
ancient Egyptians in the performance of their religious rites,
particularly in their sacrifices to the goddess Isis. This, therefore,
may be considered one of their sacred stones, whilst there is some
analogy between the cat's-eye stones and the sacred cat of the Egyptians
which recurs so often in their hieroglyphics; it is well known that our
domestic cat is not descended from the wild cat, but from the celebrated
cat of Egypt, where history records its being "domesticated" at least
thirteen centuries B.C. From there it was taken throughout Europe, where
it appeared at least a century B.C., and was kept as a pet in the homes
of the wealthy, though certain writers, speaking of the "mouse-hunters"
of the old Romans and Greeks, state that these creatures were not the
Egyptian cat, but a carniverous, long-bodied animal, after the shape of
a weasel, called "marten," of the species the "beech" or "common" marten
(_mustela foina_), found also in Britain to-day. It is also interesting
to note that the various superstitions existing with regard to the
different varieties and colours of cats also exist in an identical
manner with the corresponding colours of the minerals known as "cat's
eye."
Several varieties of cat's-eye have already been described. Another
important variety is that of the chrysoberyl called "cymophane." This is
composed of glucina, which is glucinum oxide, or beryllia, BeO, of which
there is 19.8 per cent., and alumina, or aluminium oxide, Al_{2}O_{3},
of which there is 80.2 per cent. It has, therefore, the chemical
formula, BeO,Al_{2}O_{3}. This stone shows positive electricity when
rubbed, and, unlike the sapphires described in the last chapter, which
lose their colour when heated, this variety of chrysoberyl shows no
change in colour, and any electricity given to it, either by friction or
heat, is retained for a long time. When heated in the blowpipe alone it
remains unaltered, that is, it is not fusible, and even with microcosmic
salt it requires a considerably long and fierce heat before it yields
and fuses, and acids do not act upon it. It crystallises in the 4th
(rhombic) system, and its lustre is vitreous.
The cymophane shows a number of varieties, quite as many as the
chrysoberyl, of which it is itself a variety, and these go through the
gamut of greens, from a pale white green to the stronger green of
asparagus, and through both the grey and yellow greens to dark. It is
found in Ceylon, Moravia, the Ural Mountains, Brazil, North America,
and elsewhere. The cat's-eye of this is very similar to the quartz
cat's-eye, but a comparison will make the difference so clear that they
could never be mistaken, apart from the fact that the quartz has a
specific gravity considerably lower than the chrysoberyl cat's-eye,
which latter is the true cat's-eye, and the one usually understood when
allusion is made to the stone without any distinguishing prefix, such as
the ruby, sapphire, quartz, etc., cat's eye. It should, however, be
mentioned that this stone is referred to when the names Ceylonese and
Oriental cat's-eye are given, which names are used in the trade as well
as the simple appellation, "cat's eye." One peculiarity of some of these
stones is that the "fire" or "glow" is usually altered in colour by the
colour of the light under which it is seen, the change of colour being
generally the complementary. Thus, a stone which in one light shows red,
in another will be green; the "eye" showing blue in one light will
become orange in another; whilst the yellow of another stone may show a
decided purple or amethyst in a different light.
A good test for this, and indeed most precious stones, is that they
conduct heat more quickly than does glass, and with such rapidity that
on breathing upon a stone the warmth is conducted instantly, so that,
though the stone is dimmed the dimness vanishes at once, whereas with
glass the film of moisture fades but slowly in comparison.
_The Topaz._
The name topaz is derived from the Greek _topazos_, which is the name of
a small island situated in the Gulf of Arabia, from whence the Romans
obtained a mineral which they called topazos and topazion, which mineral
to-day is termed chrysolite. The mineral topaz is found in Cornwall and
in the British Isles generally; also in Siberia, India, South America
and many other localities, some of the finest stones coming from Saxony,
Bohemia, and Brazil, especially the last-named. The cleavage is perfect
and parallel to the basal plane. It crystallises in the 4th (rhombic)
system; in lustre it is vitreous; it is transparent, or ranging from
that to translucent; the streak is white or colourless. Its colour
varies very much--some stones are straw-colour, some are grey, white,
blue, green, and orange. A very favourite colour is the pink, but in
most cases this colour is not natural to the stone, but is the result of
"burning," or "pinking" as the process is called technically, which
process is to raise the temperature of a yellow stone till the yellow
tint turns to a pink of the colour desired. The topaz is harder than
quartz, as will be seen on reference to the "Hardness" table, and is
composed of a silicate of aluminium, fluorine taking the place of some
of the oxygen. Its composition averages 16.25 per cent. of silica, 55.75
per cent. of alumina, or oxide of aluminium, and fluoride of silicium,
28 per cent. Its formula is [Al(F,OH)]_{2} SiO_{4}, or (AlF)_{2}SiO_{4}.
From this it will be understood that the fluorine will be evolved when
the stone is fused. It is, however, very difficult to fuse, and alone it
is infusible under the blowpipe, but with microcosmic salt it fuses and
evolves fluorine, and the glass of the tube in the open end of which the
stone is fixed is bitten with the gas.
Such experiments with the topaz are highly interesting, and if we take a
little of the powdered stone and mix with it a small portion of the
microcosmic salt, we may apply the usual test for analysing and proving
aluminium, thus: a strongly brilliant mass is seen when hot, and if we
moisten the powder with nitrate of cobalt and heat again, this time in
the inner flame, the mass becomes blue. Other phenomena are seen during
the influence of heat. Some stones, as stated, become pink on heating,
but if the heating is continued too long, or too strongly, the stone is
decoloured. Others, again, suffer no change, and this has led to a
slight difference of opinion amongst chemists as to whether the colour
is due to inorganic or organic matter. Heating also produces
electricity, and the stone, and even splinters of it, will give out a
curious phosphorescent light, which is sometimes yellow, sometimes blue,
or green. Friction or pressure produces strong electrification; thus the
stones may be electrified by shaking a few together in a bag, or by the
tumbling of the powdered stone-grains over each other as they roll down
a short inclined plane. The stones are usually found in the primitive
rocks, varying somewhat in different localities in their colour; many of
the Brazilian stones, when cut as diamonds, are not unlike them.
In testing, besides those qualities already enumerated, the crystalline
structure is specially perfect and unmistakable. It is doubly
refractive, whereas spinel and the diamond, which two it closely
resembles, are singly refractive. Topaz is readily electrified, and, if
perfect at terminals, becomes polarised; also the commercial solution
of violets, of which a drop only need be taken for test, is turned green
by adding to it a few grains of topaz dust, or of a little splinter
crushed to fine powder.
_The Beryl._
The beryl is a compound of silicates of beryllia and alumina, with the
formula 3BeOSiO_{2} + Al_{2}O_{3},3SiO_{2}, or
3BeO,Al_{2}O_{3},6SiO_{2}. It differs very little indeed from the
emerald, with the exception of its colour. In the ordinary varieties
this is somewhat poor, being mostly blue, or a dirty or a greenish
yellow; the better kinds, however, possess magnificent colour and
variety, such as in the aquamarine, emerald, etc. The cleavage is
parallel to the basal plane. Its lustre is sometimes resinous, sometimes
vitreous, and it crystallises in the 2nd (hexagonal) system. It occurs
in somewhat long, hexagonal prisms, with smooth, truncated planes, and
is often found in granite and the silt brought down by rivers from
granite, gneiss, and similar rocks. It is found in Great Britain and in
many parts of Europe, Asia, and America, in crystals of all sizes, from
small to the weight of several tons. The common kinds are too opaque and
colourless to be used as gems and are somewhat difficult of fusion under
the blowpipe, on the application of which heat some stones lose their
colour altogether, others partly; others, which before heating were
somewhat transparent, become clouded and opaque; others suffer no change
in colour, whilst some are improved. In almost every case a slight
fusion is seen on the sharp edges of fractures, which become smooth,
lose their sharpness, and have the appearance of partly fused glass.
The hardness varies from 7-1/4 to 8, the crystals being very brittle,
breaking with a fracture of great unevenness. The better varieties are
transparent, varying from that to translucent, and are called the
"noble" beryls. Transparent beryl crystals are used by fortune-tellers
as "gazing stones," in which they claim to see visions of future events.
_The Emerald._
Considering the particular emerald which is a variety of beryl--although
the name emerald in the trade is applied somewhat loosely to any stone
which is of the same colour, or approaching the colour of the beryl
variety--this emerald only differs chemically from the beryl, just
described, in possessing an addition of oxide of chromium. In shape,
crystallisation, fracture and hardness, it is the same, and often
contains, in addition to the chromium, the further addition of traces of
carbonate of lime, magnesia, and occasionally faint traces of hornblende
and mica, which evidently result from its intimate association with the
granite rock and gneiss, amongst which it is mostly found, the latter
rocks being of a slaty nature, in layers or plates, and, like granite,
containing mica, pyrites, felspar, quartz, etc.
Emeralds have been known from very early times, and are supposed to have
been found first in the mines of ancient Egypt. They were considered
amongst the rarest and the most costly of gems, and it was the custom,
when conferring lavish honour, to engrave or model emeralds for
presentation purposes. Thus we find Pliny describes Ptolemy giving
Lucullus, on his landing at Alexandria, an emerald on which was
engraved his portrait. Pliny also relates how the short-sighted Nero
watched the fights of gladiators through an eye-glass made of an
emerald, and in ancient times, in Rome, Greece, and Egypt, eye-glasses
made of emeralds were much valued. Many of these, as well as engraved
and carved emeralds, have been discovered in ruins and tombs of those
periods.
The copper emerald is rare; it is a hydrous form of copper silicate,
CuOSiO_{2} + H_{2}O, of a beautiful emerald green, varying from
transparent to translucent. It exhibits double refraction, and is a
crystallised mineral, brittle, and showing a green streak. This is less
hard than the real emerald, is heavier, deeper in colour, and is usually
found in crystals, in cavities of a particular kind of limestone which
exists at Altyn-Tübe, a hill in the Altai Mountains, in the Urals, and
in North and Central America.
_The Tourmaline._
The tourmaline is a most complex substance; almost every stone obtained
has a different composition, some varying but slightly, with mere traces
of certain constituents which other stones possess in a perceptible
degree. Consequently, it is not possible to give the chemical formula,
which might, and possibly would, be found but seldom, even in analyses
of many specimens. It will therefore be sufficient to state the average
composition, which is:--ferrous oxide, manganous oxide, potash, lime,
boracic acid, magnesia, soda, lithia, and water. These form, roughly
speaking, 25 per cent. of the bulk, the remainder being oxide of silicon
and oxide of aluminium in about equal parts. It crystallises in the 2nd
(hexagonal) system, with difficult cleavage and vitreous lustre.
It will naturally be expected that a substance of such complexity and
variety of composition must necessarily have a corresponding variety of
colour; thus we find in this, as in the corundum, a wonderful range of
tints. The common is the black, which is not used as a gem. Next come
the colourless specimens, which are not often cut and polished, whereas
all the transparent and coloured varieties are in great demand. To
describe adequately their characteristics with relation to light would
alone require the space of a complete volume, and the reader is referred
to the many excellent works on physics (optics) which are obtainable.
This stone is doubly refracting, exhibiting extremely strong dichroism,
especially in the blue and the green varieties. It polarises light, and
when viewed with the dichroscope shows a remarkable variety of twin
colours. It will be remembered that in Hogarth's "Rake's Progress," the
youth is too engrossed in the changing wonders of a tourmaline to notice
the entrance of the officers come to arrest him.
CHAPTER XV.
VARIOUS PRECIOUS STONES--_continued_.
_Zircon._
Zircon appears to have been first discovered by Klaproth in 1789, in the
form of an earth, and six years later he found that the stone hyacinth
contained a similar substance, both having the formula, ZrSiO_{4}, and
both having as their colouring agent ferric oxide. There are several
methods of obtaining the metallic element, zirconium; it is however with
the silicate of zirconium that we have to deal at the moment. This is
called zircon, ZrSiO_{4}, or hyacinth when transparent or red, but when
smoke-coloured, or colourless, it is the jargoon, or jarcon, and is
found in silt and alluvial soils, limestone, gneiss, and various forms
of schist, in India, Australia, the Urals, and certain parts of America.
It is often combined with and found in juxtaposition to gold and certain
varieties of precious stones. The lines of cleavage are parallel to the
sides of the prism, and the crystals have an adamantine, or diamond
lustre, varying from the completely opaque to the transparent. In some
varieties the oxide of uranium is also present in traces. It
crystallises in the 3rd (tetragonal) system, with indistinct cleavage.
Its specific gravity varies from 4.70 to 4.88, according to the specimen
and the locality.
This stone, like some of the others described, has a very wide range of
colour, going through reds, browns, greens, yellows, oranges, whites,
greys, blues from light to indigo, notwithstanding which it is somewhat
difficult to imitate scientifically, though its composition of 33 per
cent. of silica with 67 per cent. of zirconia (the oxide of zirconium),
is practically all it contains, apart from the colouring matter, such as
the metallic oxides of iron, uranium, etc. Its hardness is 7-1/2,
consequently it is untouched by a file, and so far, if one or perhaps
two of the three qualities of colour, hardness, and specific gravity,
are obtained in a chemically made zircon, the third is wanting. Under
the blowpipe, zircons are infusible, but the coloured stones when heated
strongly become heavier, and as they are contracting, their colour
fades, sometimes entirely, which changes are permanent, so that as they
possess the adamantine lustre, they are occasionally cut like a diamond,
and used as such, though their deficiency in fire and hardness, and
their high specific gravity, make them readily distinguishable from the
diamond.
On exposure to light the coloured zircon becomes more or less
decoloured; especially is this so in sunlight, for when the direct rays
of the sun fall upon it, the colours fade, and for a moment or two
occasional phosphorescence follows, as is the case when the stone is
warmed or heated in a dark room. The stone appears to be very
susceptible to brilliant light-rays, and in certain specimens which were
split for testing, one half of each being kept excluded from light for
purposes of comparison, it was found that sunshine affected them most;
then brilliant acetylene gas, which was more effective still when tinted
yellow by being passed through yellow glass. The electric arc was not so
effective, but the electric light of the mercury-vapour lamp, though
causing little change at the first, after a few hours' exposure rapidly
bleached certain of the colours, whilst having no effect on others. Coal
gas with incandescent fibre mantle was slightly effective, whilst the
coal-gas, burned direct through an ordinary burner, affected very few of
the colours, even after twenty-four hours' exposure at a distance of
three feet. In all these cases, though the colours were slightly
improved by the stones being kept for a time in the dark, they failed to
recover their original strength, showing permanent loss of colour.
_The Silicates._
The chief of these are the garnets, crystallising in the cubic system,
and anhydrous. The garnet is usually in the form of a rhombic
dodecahedron, or as a trisoctahedron (called also sometimes an
icosatetrahedron), or a mixture of the two, though the stones appear in
other cubic forms. In hardness they vary from 6-1/2 to 8-1/2. They
average from 40 to about 42 per cent. of silica, the other ingredients
being in fairly constant and definite proportions. They are vitreous and
resinous in their lustre and of great variety of colour, chiefly amongst
reds, purples, violets, greens, yellows and blacks, according to the
colouring matter present in their mass. There are many varieties which
are named in accordance with one or more of their constituents, the best
known being: (A) The iron-alumina garnet, having the formula 6FeO,
3SiO_{2} + 2Al_{2}O_{3}, 3SiO_{2}. This is the "precious" garnet, or
almandine, sometimes called the "Oriental" garnet; these stones are
found in Great Britain, India, and South America, and are deep red and
transparent, of vitreous lustre. They get up well, but certain varieties
are so subject to defects in their substance, brought about by pressure,
volcanic action, and other causes, some of which are not yet known, that
their quality often becomes much depreciated in consequence. This
inferior variety of the iron-alumina garnet is called the "common"
garnet, and has little lustre, being sometimes opaque. The perfect
qualities, or almandine, as described above, are favourite stones with
jewellers, who mount great quantities of them.
The second variety is the (B) lime-iron garnet, formula, 6CaO,3SiO_{2} +
2Fe_{2}O_{3},3SiO_{2}. The chief of this class is the melanite,
sometimes dull, yet often vitreous; it is mostly found in volcanic
rocks, such as tuff; this variety is very popular with jewellers for
mourning ornaments, for as it is a beautiful velvet-black in colour and
quite opaque, it is pre-eminent for this purpose, being considerably
less brittle than jet, though heavier. Another variety is the
"topazolite," both yellow and green. The "aplome" is greenish-yellow,
yellowish-green, brown, and usually opaque. A further form of lime-iron
garnet is the "pyreneite," first found in the Pyrenees Mountains, hence
its name.
The (C) lime-chrome garnets--6CaO,3SiO_{2} + 2Cr_{2}O_{3}, 3SiO_{2}--the
chief of which is "uwarowite." This is of a magnificent emerald green
colour, translucent at edges and of a vitreous lustre. When heated on
the borax bead it gives an equally beautiful green, which is, however,
rather more inclined to chrome than emerald. This is an extremely rare
stone in fine colour, though cloudy and imperfect specimens are often
met with, but seldom are large stones found without flaws and of the
pure colour, which rivals that of the emerald in beauty.
The fourth variety (D) is the lime-alumina garnet, its formula
being--6CaO,3SiO_{2} + 2Al_{2}O_{3},3SiO_{2}. Like the others, it has a
number of sub-varieties, the chief being the "cinnamon stone," which is
one of great beauty and value when perfect. This stone is almost always
transparent when pure, which property is usually taken as one of the
tests of its value, for the slightest admixture or presence of other
substances cloud it, probably to opacity, in accordance with the
quantity of impurity existent. This variety is composed of the oxides of
aluminium and silicon with lime. In colour it ranges from a beautiful
yellowish-orange deepening towards the red to a pure and beautiful red.
"Romanzovite" is another beautiful variety, the colour of which ranges
through browns to black. Another important variety is the "succinite,"
which gets up well and is a favourite with jewellers because of its
beautiful, amber-like colour, without possessing any of the drawbacks of
amber.
(E) The magnesia-alumina garnet--6MgO,3SiO_{2} +
2Al_{2}O_{3},3SiO_{2}--is somewhat rare, the most frequently found being
of a strong crimson colour and transparent. This variety is called
"pyrope," the deeper and richer tints being designated "carbuncle," from
the Latin _carbunculus_, a little coal, because when this beautiful
variety of the "noble" garnet is held up between the eyes and the sun,
it is no longer a deep, blood-red, but has exactly the appearance of a
small piece of live or glowing coal, the scarlet portion of its
colour-mixture being particularly evident. The ancient Greeks called it
anthrax, which name is sometimes used in medicine to-day with reference
to the severe boil-like inflammation which, from its burning and
redness, is called a carbuncle, though it is more usual to apply the
word "anthrax" to the malignant cattle-disease which is occasionally
passed on to man by means of wool, hair, blood-clots, etc., etc., and
almost always ends fatally. A great deal of mystery and superstition has
always existed in connexion with this stone--the invisibility of the
bearer of the egg-carbuncle laid by a goldfinch, for instance.
(F) The manganese-alumina garnet--6MnO,3SiO_{2} +
2Al{2}O_{3},3SiO_{2}--is usually found in a crystalline or granular
form, and mostly in granite and in the interstices of the plates, or
laminæ, of rocks called schist. One variety of this, which is a deep
hyacinth in colour, though often of a brown-tinted red, is called
"spessartine," or "spessartite," from the district in which it is
chiefly found, though its distribution is a fairly wide one.
_The Lapis-Lazuli._
The lapis-lazuli, sometimes called "azure stone," is almost always blue,
though often containing streaks of white and gold colour, the latter of
which are due to the presence of minute specks or veins of iron pyrites,
the former and colourless streaks being due to free lime, calcite, and
other substances which have become more or less blended with the blue
colour of the stone. It has a vitreous lustre, crystallises in the 1st,
or cubic system, and is a complex substance, varying considerably in its
ingredients in accordance with the locality in which it is found, its
matrix, and the general geological formation of the surrounding
substances, which may, by the penetration of moisture, be brought to
bear upon the stone, thus influencing to a great extent its chemical
composition. So that we find the stone composed of about a quarter of
its substance of alumina, or oxide of aluminium, silica to the extent of
almost half, the remainder being lime, soda, sulphur, and occasionally
traces of copper and iron. It is mostly found in granite and certain
crystalline limestone rocks, in fairly large masses. It is of great
antiquity, figuring extensively in ancient Egyptian history, both in its
form as a stone and ground up into a pigment for the decoration of
sacred and royal vessels and appointments. When so ground, it forms the
stable and magnificent colour, _genuine_ ultramarine, which is the
finest and purest blue on the artist's palette, but owing to its
extremely high price its use is not in very great demand, especially as
many excellent chemical substitutes of equal permanence are obtainable
at little cost.
_The Turquoise._
The turquoise is a pseudomorph (see Chapter IV., "Cleavage.") In colour
it is blue or greenish-blue, sometimes opaque, varying between that and
feeble translucency, though it should be said that in all forms, even
those considered opaque, a thin cutting of the stone appears almost
transparent, so that the usual classing of it among the opaque stones
must be done with this reservation. In composition it contains about 20
per cent. of water, about a third of its substance being phosphoric
acid, or phosphorus-pentoxide; sometimes nearly half of it is alumina,
with small quantities of iron in the form of variously coloured oxides,
with oxide of manganese. The great proportion of water, which it seems
to take up during formation, is mostly obtained in the cavities of
weathered and moisture-decomposing rocks. Its average formula may be
said to be Al_{2}O_{3}P_{2}O_{5} + 5H_{2}O, and sometimes Al_{2}O_{3}
FeOP_{2}O_{5} + 5H_{2}O. It must therefore follow that when the stone is
heated, this water will separate and be given off in steam, which is
found to be the case. The water comes off rapidly, the colour of the
stone altering meanwhile from its blue or blue-green to brown. If the
heat is continued sufficiently long, this brown will deepen to black,
while the flame is turned green. This is one of the tests for turquoise,
but as the stone is destroyed in the process, the experiment should be
made on a splinter from it.
This stone is of very ancient origin, and many old turquoise deposits,
now empty, have been discovered in various places. History records a
magnificent turquoise being offered in Russia for about £800 a few
centuries ago, which is a very high price for these comparatively common
stones.
Owing to the presence of phosphorus in bones, it is not uncommon to
find, in certain caves which have been the resort of wild animals, or
into which animals have fallen, that bones in time become subjected to
the oozing and moisture of their surroundings; alumina, as well as the
oxides of copper, manganese and iron, are often washed across and over
these bones lying on the cave floor, so that in time, this silt acts on
the substance of the bones, forming a variety of turquoise of exactly
the same composition as that just described, and of the same colour. So
that around the bones there eventually appears a beautiful turquoise
casing; the bone centre is also coloured like its casing, though not
entirely losing its bony characteristics, so that it really forms a kind
of ossified turquoise, surrounded by real turquoise, and this is called
the "bone turquoise" or "odontolite."
INDEX
Adamantine lustre, 28
glimmering, 29
glinting, or glistening, 29
lustreless, 29
shining, 29
splendent, 29
Agate, 11
Almandine, 101
Amethyst, 11
oriental, 85
sapphire, 85
Amorphous stones and their characteristics, 23
Analysis, 5
Aplome, 101
Asters, or asteriated stones, 82, 87-91
Azure-stone, 103
Beryl, 10, 94
colours of, in dichroscope, 34
Beryllium, 10
Bezils, 66
Black stones, list of, 79
Blue sapphire, composition of the, 10
stones, list of, 77
Bone-turquoise, 106
Break, as opposed to cleavage, 19
Brilliant-cut stones, 66
Brown stones, list of, 76
Building up of crystals, 13
Burnt, or pinked topaz, 92
Cabochon-cut stones, 64
(the double), 65
(the hollow), 65
Carbonate series, 11
Carbuncle, 102, 103
Cat of Egypt, 89
Cat's eye stones, 82, 87-91
list of (see "Chatoyant Stones"), 78
Ceylonese cat's eye (see "Cat's eye")
Change of colour (not to be confused with "Play of colour" and "Opalescence,"
which see; see also "Fire"), 36
Characteristics of precious stones, 1, 3
Chatoyant stones, list of, 78
Chemical illustration of formation of precious stones, 8
Chloride of palladium in dichroscope, 34
Chrysoberyl, 88
Chrysolite, 11
ordinary, or "noble", 85
oriental, 85
Cinnamon stone, 102
Claims of precious stones, 4
Cleavage affecting tests, 43
and "cleavage" as opposed to "break", 19, 22
Colour, 26, 28, 30, 32
Colourless stones, list of, 75
Colours and characteristics of the various opals, 35, 36
of precious stones, list of, 75-79
Common garnet, 101
opal, 35
Composite crystals, 13
Composition of paste, or strass, for imitation stones, 71
Composition of precious stones, 7
Converted stones, 72
Corundum, 82
Crown portion of stones, 65, 66
Crystalline structure, physical properties, of 13
Crystallography, 14
Crystals, axes of symmetry, 15
groups of, 15, 16
planes of symmetry, 15
systems of, 16
(1) Cubic--isometric, monometric, regular, 16
(2) Hexagonal--rhombohedral, 16
(3) Tetragonal--quadratic, square prismatic, dimetric, pyramidal, 16
(4) Rhombic--orthorhombic, prismatic, trimetric, 16
(5) Monoclinic--clinorhombic, monosymmetric, oblique, 16, 17
(6) Triclinic--anorthic, asymmetric, 16, 17
treatment of, 14
Culasse portion of stones, 66
Cullinan diamond (see also "Stars of Africa"), 22, 64, 68, 80
Cutting of precious stones, 3, 4, 62
Cymophane, 90
Definition of a precious stone, 1
Diamond, characteristics of the, 80
composition of the, 10
(sapphire), 86
unique, 10
(zircon), 99
Diaphaneity, 26, 28
Diaphanous stones, 28
Dichroscope, 33
how to make a, 33
how to use a, 34
Dimorphism in precious stones, 25
Double cabochon-cut stones, 65
refraction (see "Refraction")
Doublets, 72
Electric and magnetic influences, 57
experiments with precious stones and pithball and electroscope, 57
experiments with tourmaline, 58, 59
Emerald, 10, 11, 95, 96
oriental, 85
En cabochon-cut stones, 64
Experiments to show electric polarity, 58, 59
Facets in stones, description of the, 67, 68
Feminine stones, 85
Fire in stones (see also "Change of Colour," "Opalescence," and "Play of
Colour"), 36, 37
Fire opal, 35
Flame-coloured stones, list of, 76
Flaws, 63
Formation of precious stones, 5, 8
chemical illustration of, 8, 9
Garnet, 11, 100
Garnets
(A) iron-alumina (called also almandine and precious
or oriental garnet), 101
sub-variety, common garnet, 101
(B) lime-iron, 101
sub-variety aplome, 101
melanite, 101
pyreneite, 101
topazolite, 101
(C) lime-chrome, 101, 102
sub-variety uwarowite, 101, 102
(D) lime-alumina, 102
sub-variety cinnamon stone, 102
romanzovite, 102
succinite, 102
(E) magnesia-alumina, 102, 103
sub-variety carbuncle, or anthrax, 102, 103
noble, 103
pyrope, 102
(F) manganese-alumina, 103
sub-variety spessartine, or spessartite, 103
Girdle portion of a stone, 66
Glimmering, in lustre, definition of, 29
Glinting, or glistening in lustre, definition of, 29
_Goutte de suif_-cut stones, 65
Great Mogul diamond, 64
Green stones, list of, 78
Groups of crystals (see "Crystals")
Hardness, physical properties of, 39
table of, 39, 40, 41
Heat indexes, 54
physical properties of, 52
Hollow-cabochon, 65
Hyacinth, ordinary (a form of zircon), 85, 98
oriental, 85
Hyalite (opal), 35
Hydrophane (opal), 35
Imitations and tests of precious stones, 70
Indigo sapphires, 86
Ink sapphires, 85
Iridescence, and cause of, 37, 38
Iron-alumina garnets, 101
Jacinth, oriental, 85
Jarcon, or jargoon, 98
Koh-i-nûr, 64
Lapis-lazuli, 103
Light, physical properties of, 26
Lime-alumina garnets, 102
cinnamon stone, 102
romanzovite, 102
succinite, 102
Lime-chrome garnets, 101, 102
uwarowite, 101, 102
Lime-iron garnets, 101
aplome, 101
pyreneite, 101
topazolite, 101
List of stones according to colour, 75-79
hardness, 39-41
specific gravity, 48-50
Lustre, 26, 28
Lustreless, definition of, 29
Lynx-eye stones, 87
Magnesia-alumina garnets, 102, 103
carbuncle, or anthrax, 102
noble, 103
pyrope, 102
Magnetic and electric influences, 57-61
Malachite, 11
Manganese-alumina garnets, 103
spessartine, or spessartite, 103
Masculine stones, 85
Melanite, 101
Menilite (opal), 36
Metallic-lustre stones, 28, 29
Mohs's table of hardness, 39-41
Noble garnet, 103
or precious opal, 35
Non-diaphanous stones, 28
Odontolite, 106
Olivine corundum (see "Chrysolite"), 85
Opal, 11
varieties of, 35, 36
Opalescence (not to be confused with "Change of Colour" and "Play of Colour,"
which see; see also "Fire"), 36, 37
Oriental amethyst, 85
cat's eye (see "Cat's eye")
emerald, 85
garnet, 101
topaz, 85
Origin of precious stones, 7
Paste, or strass, for imitation stones, composition of, 71
Pavilion portion of cut stones, 66
Pearly-lustre stones, 28, 29
Peridot (see "Noble Chrysolite"), 85
Pink-coloured stones, list of (see also Red), 77
Pinked topaz, 92
Phosphorescence, 26, 30
Physical properties:--
A.--Crystalline structure, 13
B.--Cleavage, 19
C.--Light, 26
D.--Colour, 32
E.--Hardness, 39
F.--Specific gravity, 45
G.--Heat, 52
H.--Magnetic and electric influences, 57
Play of colour (not to be confused with "Change of Colour" and "Opalescence,"
which see; see also "Fire"), 36, 37
Pleochroism, 33
Polarisation, electric, 58, 59
of light, 26, 27
Polariscope, 27, 28
Polishing precious stones, 3, 4
Polymorphism in precious stones, 25
Precious, or noble opal, 35
Pseudomorphism in precious stones, 23, 24
Pyreneite, 101
Pyro-electricity, development and behaviour of, 58-60
Pyrope, 102
Qualities of precious stones, 1, 3
Red and rose-coloured stones, list of (see also Pink), 76, 77
Reflection of light, 26, 28
Refraction of heat, 52-55
light, 26, 27
Reproduction of crystalline form, 20, 21
Resinous lustre stones, 28, 29
Rock-crystal, 11
Romanzovite, 102
Rose-coloured stones (see Red, above), 76, 77
Rose, or rosette-cut stones, 65
Rothschild's testing solution, 73
Ruby, characteristics of, 83
composition of, 10
Sapphire, amethyst, 85
and its varieties, 84, 85
cleared, 86
diamonds, 87
indigo, 86
ink, 85
the blue, composition of, 10, 85
water, 86
Semi-diaphanous stones, 28
Shining, in lustre, definition of, 29
Silica group, composition of the, 11
Silicates, 100
Silky-lustre stones, 28, 29
Single-refraction (see "Refraction")
South African diamond (see "Cullinan Diamond")
Specific gravity, 45
Splendent, in lustre, definition of, 29
Splitting of the Cullinan diamond, 22
Star-portion of stones, 65
Stars of Africa (see also "Cullinan Diamond"), 22, 64, 68
Starting or splitting of stones on cleavage planes, 23
Step-cut stones, 66
Stones arranged according to colour, 75-79
hardness, 39-41
specific gravity, 48-50
Strass for imitation stones, composition of, 71
Sub-metallic in lustre, definition of, 29
Sub-translucent stones, 28
Sub-transparent stones, 28
Succinite, 102
Synthesis, 5
Systems of crystals (see "Crystals")
Table-cut stones, 65
Tallow drops, 65
Teeth of stone, 65
Testing by crystalline structure, 17
hardness, 40, 43
with needles, 41
gems by dichroscope, 33, 34
solution (Rothschild's), 73
Tests of precious stones (general), 70
Topaz, 11, 91
colours of, in dichroscope, 34
oriental, 85
Topazolite, 101
Tourmaline, 96, 97
electric experiments with, 58, 59
Translucent stones, 28
Transmission of heat, 52-56
light, 26
Transparent stones, 28
Trap-cut stones, 66
Tri-morphism in precious stones, 25
Triplets, 72
Turquoise, 104
(bone), 106
composition of the, 11
odontolite, 106
Uwarowite, 101, 102
Violet stones, list of, 78
Vitreous-lustre stones, 28, 29
Water-sapphires, 86
White (paste) stones, 71
stones, list of, 75
Yellow stones, list of, 76
topaz, 92
Zircon, 10, 98
diamonds, 99
Zirconium, 10
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