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Title: Gems in the Smithsonian Institution
Author: Desautels, Paul E.
Language: English
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*** Start of this LibraryBlog Digital Book "Gems in the Smithsonian Institution" ***
[Illustration: Faceted, egg-shaped, 7000-carat rock crystal from
Brazil. The gold stand is inset mostly with Montana sapphires. The
gem was cut and the stand was designed and constructed by Capt. John
Sinkankas of California. (7¼ inches high in all.)]
_Gems_
_in the_
SMITHSONIAN
INSTITUTION
by PAUL E. DESAUTELS
_Associate Curator_
Division of Mineralogy
WASHINGTON, D. C.
1965
[Illustration: FOR THE INCREASE AND DIFFVSION OF KNOWLEDGE AMONG
MEN • SMITHSONIAN INSTITVTION • WASHINGTON 1846]
SMITHSONIAN
INSTITUTION
PUBLICATION
No. 4608
LIBRARY OF CONGRESS
Card No. 65-60068
CONTENTS
The National Gem Collection 1
The Study of Gems 3
The Shaping of Gemstones 10
Gem Substitutes 20
Gem Lore 24
The Principal Gem Species 27
Some Notable Gems in the Collection 70
[Illustration: Prof. F. W. Clarke, former honorary curator of the
Division of Mineralogy who assembled the Smithsonian Institution’s
first gem collection in 1884.]
[Illustration: Dr. Isaac Lea, Philadelphia gem collector whose
collection was the nucleus around which the Smithsonian
Institution’s gem collection has been built through the years.]
[Illustration: Dr. Leander T. Chamberlain, son-in-law of Dr. Isaac
Lea, who became honorary curator of the Smithsonian Institution’s
gem collection in 1897. Income from his bequest is used to purchase
gems for the Isaac Lea gem collection.]
1
THE NATIONAL GEM COLLECTION
Man has been using certain mineral species for personal adornment since
prehistoric times. However, of the almost 2000 different mineral
species, relatively few, perhaps only 100, have been used traditionally
as gems. To be used as a gem, a mineral species must have durability as
well as beauty. Lack of durability eliminates most minerals as gems,
although some relatively fragile gem materials such as opal are prized
because of their exceptional beauty. Actually, some gem materials are
not minerals at all. Pearl, amber, jet, and coral are formed by living
organisms.
In the National Gem Collection, the Smithsonian Institution has
assembled a large representation of all known gem materials. The display
portion of the collection consists of more than 1000 items selected to
illustrate the various kinds of gems and to show how their beauty is
enhanced by cutting and polishing. All of these gems are gifts of
public-spirited donors who, by giving the gems directly or by
establishing endowments for their purchase, have contributed to the
enjoyment of the many thousands of persons who visit the Smithsonian
Institution each week.
The National Gem Collection had its beginning in 1884 when Prof. F. W.
Clarke, then honorary curator of the Division of Mineralogy, prepared an
exhibit of American precious stones as a part of the Smithsonian
Institution’s display at the New Orleans Exposition. The same collection
was displayed at the Cincinnati Exposition the following year. Between
1886 and 1890 the growth of the collection was slow, but in 1891 most of
the precious stones collected by Dr. Joseph Leidy of Philadelphia were
obtained, and these, combined with those already on hand, were exhibited
at the World’s Columbian Exposition at Chicago in 1893.
Great stimulus was given the collection in 1894 when Mrs. Frances Lea
Chamberlain bequeathed the precious stones assembled by her father, Dr.
Isaac Lea. Her husband, Dr. Leander T. Chamberlain, who in 1897 became
honorary curator of the collection, contributed a large number of
specimens and, upon his death, left an endowment fund. The income from
that fund has been used to steadily increase the collection over the
years. Extremely rare and costly gems suitable for exhibition are beyond
the income derived from the Chamberlain endowment, but this gap has been
filled by many important donations, the most notable being the gift of
the Hope Diamond by Harry Winston, Inc., New York City. Thus, from
modest beginnings in 1884, there has been accumulated the magnificent
collection of gems belonging to the people of the United States. The
collection is displayed in the Smithsonian Institution’s great Museum of
Natural History.
[Illustration: Left to right: 42-carat brazilianite, 8.4-carat
euclase, 7.6-carat benitoite, 12-carat willemite, 20-carat
amblygonite, and 16-carat orthoclase. (About two-thirds actual
size.)]
2
THE STUDY OF GEMS
To the average person it might seem that a jeweler’s showcase of gems
presents innumerable kinds of precious stones, when actually only a few
species of minerals are there. Perhaps only diamond, ruby, emerald,
aquamarine, sapphire, opal, tourmaline, and amethyst would comprise the
entire stock. Yet, since the mineral kingdom consists of about 2000
distinct species, it would seem that a few more kinds of gemstones would
be available. Certainly, many more minerals than are seen displayed by
the jeweler have been used as gems over the centuries. The study of all
these species of gem minerals constitutes modern gemology—a specialized
branch of the science of mineralogy.
With the few exceptions already noted, all gems are minerals found in
the earth’s crust. A mineral is a natural substance having a definite
chemical composition and definite physical characteristics by which it
can be recognized. However, for a mineral to qualify as a gem it must
have at least some of the accepted requirements—brilliance, beauty,
durability, rarity, and portability. Of course, if a gemstone happens to
be “fashionable” it will have additional importance. Rarely does a
single gem possess all of these qualities. A fine-quality diamond,
having a high degree of brilliance and fire, together with extreme
hardness and great rarity, comes closest to this ideal, and in the world
of fashion the diamond is unchallenged among gems. The opal, by
contrast, is relatively fragile, and it depends mainly on its rarity and
its beautiful play of colors to be considered gem material.
When a gem material, as found in nature, has at least a minimum number
of the necessary qualities, it is then the task of the lapidary, or gem
cutter, to cut it and polish it in such a way as to take greatest
advantage of all its possibilities for beauty and adornment.
PHYSICAL CHARACTERISTICS OF GEMSTONES
When a gemologist or a gem cutter examines an unworked mineral fragment
(called _rough_) he looks for certain distinguishing characteristics
that will aid him in identifying the mineral and in determining the
procedures he should use in cutting it.
Scale of Hardness
Soft 1. Talc
^ 2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Feldspar
7. Quartz
8. Topaz
v 9. Corundum
Hard 10. Diamond
It is difficult to list these characteristics in the order of
importance, but _hardness_ would rank high. Hardness of a gem is best
defined as its resistance to abrasion or scratching. Most commonly used
for comparison is the Mohs scale, which consists of selected common
minerals arranged in the order of increasing hardness. On this scale,
topaz is rated as 8 in hardness, ruby as 9, and diamond, the hardest
known substance, as 10. Any gem with a hardness less than that of
quartz, number 7 in the scale, is unlikely to be sufficiently
scratch-resistant for use as a gem. A less precise scale, using common
objects for comparison, might include the fingernail with a hardness up
to 2½, a copper coin up to 3, a knife blade to 5½, a piece of window
glass at about 5½, and a steel file between 6 and 7, depending on the
type of steel. By this scale, any stone that remains unmarred after
being scraped by a piece of window glass will have a hardness greater
than 5½. The more important gemstones—which include diamond, ruby,
sapphire, and emerald—all have a hardness much greater than 5½.
The size of a gemstone usually is indicated by its _weight_ in carats.
The expression “a 10-carat stone” has meaning—if somewhat inexact—even
to the nonexpert. Specifically, a carat is one-fifth of a gram, which is
a unit of weight in the metric system small enough so that approximately
28 grams make an ounce. A 140-carat gemstone, then, weighs about an
ounce.
Another distinguishing characteristic of a gemstone is its specific
gravity, which is an expression of the relationship between the stone’s
own weight and the weight of an equal volume of water. We are aware of a
difference in weight when we compare lead and wood, yet it would not
always be correct to say that lead weighs more than wood, for a large
piece of wood can weigh more than a small piece of lead. Only by
comparing equal volumes of these materials can the extent of the weight
difference be clear and unmistakable. Diamond is 3½ times heavier than
the same volume of water, so its specific gravity is 3.5. Since each
species of gem has its own specific gravity, which can be determined
without harming the stone, this standard of comparison is a valuable aid
in identifying gems. Several techniques have been devised for
determining specific gravity, and most of them make use of some kind of
weighing device or balance.
Among the most striking and useful of the distinguishing characteristics
of gemstones are those that involve the effects on light.
An important effect of a gem on light is the production of color, upon
which many gems depend for their beauty. Some gem materials, such as
lapis lazuli, have little to offer except color. Many gemstones vary
widely in color, owing to the presence of varying but extremely small
amounts of impurities. Thus, the gemstone beryl may occur as blue-green
(aquamarine), as pink (morganite), as rich green (emerald), as yellow
(golden beryl), or even colorless (goshenite).
[Illustration: Sketch of a simple balance used to determine specific
gravity of a gemstone. The operator places the gemstone in the upper
pan (A), moves the weight (B) along the beam (C) until it balances
perfectly, and notes the number at the weight’s position. He then
transfers the gemstone to the lower pan (D), which is completely
immersed in water, and moves the weight along the beam to restore
balance. He notes the scale number at the new position and
determines the specific gravity simply by dividing the first number
by the difference between the two numbers. If the gemstone is large,
the operator can use heavier sliding weights. (E).]
Gemstones such as beryl and sapphire that depend on impurities for their
color are said to be _allochromatic_; others, such as peridot and
garnet, which are highly colored even when pure, are said to be
_idiochromatic_. The color of a gem is further described according to
its _hue_, _tint_, and _intensity_. Hue refers to the kind of color,
such as red, yellow, green, etc.; tint refers to the lightness or
darkness of the hue; and intensity refers to vividness or dullness.
Throughout history, the most popular colored stones have been those with
hues of red, green, or blue of dark tint and high intensity.
[Illustration: A 43-carat albite from Burma (at left), 76-carat
tourmaline from Brazil, and 30-carat wernerite from Burma exhibit a
strong cat’s-eye effect because of reflection from inclusions in
parallel arrangement within the stones. (Actual size.)]
[Illustration: Asterism (star effect) is caused by parallel
inclusions arranged in several directions related to the crystal
structure of the gemstone. Two rays in the 175-carat, 6-rayed star
garnet from Idaho (at left in photo) are weaker than the other four
because of fewer inclusions in that direction. The 23-carat star
orthoclase from Ceylon shows brightly all of its four possible rays.
(Actual size.)]
The effect of a gem on light may be more than the production of color.
Several of the so-called phenomenal stones are prized for other effects.
Holes, bubbles, and foreign particles, when properly aligned in parallel
groupings, can produce interesting light effects. The play of colors of
opal and labradorite, the _chatoyancy_ or silky sheen of tiger’s-eye and
cat’s-eye, the _opalescence_ or pearly reflections of opal and
moonstone, and the _asterism_ or star effect of rubies and sapphires are
caused by the reaction of light to minute _inclusions_ or imperfections
in the gemstone.
When light passes into or through a gemstone with little or no
interruption, the stone is said to be transparent, as opposed to a stone
through which light passes with greater difficulty, and which is said to
be either translucent or opaque, depending on the degree of light
interruption.
[Illustration: Rays of light passing into a gemstone are refracted
(bent) in varying amounts depending on the gem species and also on
the angle at which the light strikes the stone. The light rays are
reflected back toward the top of the stone by internal faces
(facets), and they are refracted again as they leave.]
[Illustration: How a gem refractometer, a simple device to operate,
is used to measure quickly the refractive index of a cut gemstone. A
light beam passing through the opening (A) is reflected from the
table of a gemstone (G) through a lens system (L) and, by prism (P),
into the eye of the observer (E). The maximum angle of reflection
(N), which depends on the refractive index of the gemstone, controls
the angle at which the beam comes through the eyepiece (EP). The
refractive index is read directly from a scale in the eyepiece.]
The action of a gemstone upon the light which strikes its surface and is
either reflected or passed through it sometimes results in highly
desirable effects that enhance its beauty and aid in its identification.
Light passing into a stone is bent from its path, and the amount of
bending (_refraction_) depends upon the species of the gemstone. When
the degree of bending can be measured, the gem species can be
identified, since very few species of gemstones bend light to exactly
the same degree. An instrument called a gem refractometer is used to
determine the degree to which cut stones refract, or bend, light. The
measurement obtained is the _refractive index_ of the gemstone.
Many gemstones can split a beam of light and bend one part more than the
other, thus producing _double refraction_, or two different measurements
of refractive index.
[Illustration: When a ray of ordinary white light enters some
gemstones it is dispersed (split up) into rays of the separate
colors of which it is composed. These rays are reflected inside the
gem and are further separated by additional refraction as they leave
the gemstone. This dispersion accounts for the colored flashes of
light, or fire, for which diamond is highly prized.]
Gems have the ability to separate “white light” (the mixture of all
colors) into its various colors, producing flashes of red, yellow,
green, and other colors. Separation occurs because the various colors,
or wavelengths composing white light passing through the gem, are each
bent or refracted a different amount. Red is bent least, followed in
order by orange, yellow, green, blue, and violet, which is bent most.
This characteristic of being able to produce flashes of color, as seen
prominently in diamond, is known as _dispersion_ or _fire_. Quartz and
glass have low dispersion, and hence they make poor diamond substitutes.
Some of the newer synthetic gemstones, such as titania, have extremely
high dispersion, with resulting fire. Zircon, a natural gemstone of
suitable hardness, exhibits high dispersion and is a commonly used
substitute for diamond.
CHEMICAL CHARACTERISTICS OF GEMSTONES
Since gems are embraced in the mineral kingdom, and minerals are
naturally occurring chemical substances, it follows that all the
accepted terms of chemical description can be applied to them. When a
chemist learns that ruby is an impure aluminum oxide, he understands a
great deal about the nature, origin, and behavior of ruby. He can assign
to it the chemical formula Al₂O₃, symbolizing its basic composition as
two atoms of aluminum united with three of oxygen. Similarly, other
popular gemstones can be described chemically as follows:
Diamond Carbon C
Sapphire Aluminum oxide Al₂O₃
Quartz Silicon dioxide SiO₂
Emerald Beryllium aluminum silicate Be₃Al₂(SiO₃)₆
Spinel Magnesium aluminate Mg(AlO₂)₂
Significantly, ruby and sapphire are chemically identical, both being of
the mineral species corundum. As already explained, the difference in
color is due entirely to very slight traces of chemical impurities.
Frequently, the impurities are present in irregular patches that give
spotty color effects.
Some mineral species possess many of the desirable qualities of
gemstones yet cannot be used as gems because they are chemically active
and therefore are less durable. They undergo alteration and
decomposition when exposed to light or to one or another of such
substances as air, water, skin acids and oils.
3
THE SHAPING OF GEMSTONES
Gemstone crystals often have naturally brilliant, reflecting faces, but
rarely are they perfect and unblemished. Also, their natural shapes do
not provide the best expression of their luster, brilliance, dispersion,
color, and other inherent properties. In fashioning a gemstone, the
skilled artisan tries to develop these hidden assets and to otherwise
enhance the gemstone’s general beauty.
From ancient times until the 1600’s little was attempted in the way of
shaping gemstones other than to smooth or polish the natural form.
Although similarly smoothed, or _tumbled_, gemstones recently have
returned to fashion, the finest pieces of gem rough are now converted
mainly into _faceted_, or shaped, stones. Standard types of facets—the
flat faces that are ground and polished on the rough gem material—have
been given individual and group names. A typical example is the
_brilliant_ cut, which is most commonly used to best bring out the
qualities of a diamond.
[Illustration: The standard brilliant cut, with a pattern of many
facets, is commonly used for gemstones having a high refractive
index and, therefore, great brilliance.]
[Illustration: Characteristic of the standard brilliant cut are the
32 crown facets surrounding a relatively small, flat, table facet
and the 24 pavilion facets and culet at the bottom of the stone.]
[Illustration: Ideal proportions for the standard brilliant cut have
been carefully determined so that the maximum amount of light will
be reflected back out the top of the stone. Incorrect proportions
cause the light to be lost at the bottom of the stone.]
[Illustration: The step cut, often called the emerald cut,
frequently is used for colored stones because the large table
permits a good view of the color.]
[Illustration: The emerald or step cut provides a large table and a
full bottom for the stone. Although the number of crown and pavilion
facets may vary, the general pattern is maintained.]
[Illustration: The simplified English brilliant cut takes maximum
advantage of the strong dispersion of diamond, with its flashes of
fire, but the fewer facets provide less sparkle than the standard
brilliant cut.]
The diagram shows a brilliant-cut diamond with angles and facets
arranged to give the stone maximum internal reflection as well as to
make use of its strong dispersive ability. Certain of the light beams
passing into a brilliant-cut diamond produce colorless brilliance by
being reflected back out of the stone through the _table_ by which they
entered. Other light beams, emerging through inclined facets, are split
up by dispersion into the rainbow, or fire, effect so prized in
diamonds. A stone that has been cut too wide for its depth, with
incorrect facet angles, will look large for its weight but its
brilliance and fire will have been drastically reduced.
[Illustration: The English brilliant cut has 28 crown and pavilion
facets—28 fewer than the standard brilliant cut.]
[Illustration: The Dutch rose cut is a very simple one that is used
mainly for small diamonds in jewelry that features a larger, colored
stone. It is based on a form that originated in India and was
introduced through Venice.]
For other purposes and for other kinds of precious stones a number of
basic cuts have been developed. The _brilliant_ and _step_ cuts are by
far the commonest of these basic cuts, but modern jewelry design
frequently uses such fancy cuts as the baguette, cut-corner triangle,
epaulet, half moon, hexagon, keystone, kite, lozenge, marquise,
pentagon, square, trapeze, and triangle. Some of these are shown here.
[Illustration: Just as the English brilliant cut, because of its 28
fewer facets, has less sparkle than the standard brilliant cut, the
step brilliant, with its 20 additional facets, has greater sparkle.]
[Illustration: The step brilliant cut is a complicated modification
of the standard brilliant. With an additional 12 facets in the crown
and 8 in the pavilion, the step brilliant has 78 facets, compared
with the 58 of the standard.]
[Illustration: Various kinds of cuts have been devised for special
purposes in jewelry design. These include the pentagon (1), lozenge
(2), hexagon (3), cut-corner triangle (4), kite (5), keystone (6),
epaulet (7), baguette (8), trapeze (9) and square (10).]
[Illustration: With this typical trim saw, water is used as a
coolant for the rapidly rotating metal disk, which has a
diamond-impregnated rim. Here, the blade is cutting its way through
a piece of gem tourmaline.]
In general, there are three operations in preparing a gemstone from the
rough—sawing, grinding, and polishing. Sawing usually is accomplished by
using a thin, diamond-impregnated, rapidly rotating disk of soft iron or
bronze, with oil or water being used as a coolant. The very hard diamond
dust literally scratches its way through the stone. Once the stone is
sawed to shape, the facets are ground and polished on a rotating
horizontal disk by the use of various abrasives. For rough grinding,
silicon carbide—or sometimes diamond powder—is used. Scratches are
removed and a high polish is given by the use of tin oxide, pumice,
rouge, or other fine-grained abrasives. The thick disks, or laps, are
made of cast iron, copper, lead, pewter, wood, cloth, leather, and
certain other materials. Since each species of gemstone differs in its
characteristics, each must be treated somewhat differently as to sawing
and lapping speeds, kind of lap, and choice of abrasives. Because of the
greatly increased interest in gem cutting as a hobby and the large
number of amateur cutters, a substantial market has developed in the
United States for lapidary supplies and equipment. New kinds of
machinery, new abrasives, and new kinds of saws and laps are introduced
regularly. Fundamentally, however, the process still involves sawing,
grinding, and polishing.
[Illustration: The final step in preparing a gemstone from rough is
the applying of a high polish by pressing the stone against a
rotating disk that has an extremely fine abrasive on its surface.
Here, the disk is of felt, and the abrasive is tin oxide.]
[Illustration: The cabochon cut gets its name from the French word
“caboche,” meaning pate or knob, a reference to the rounded top of
the stone. Here, from top to bottom, beginning at left, are
cabochons of turquoise, agate, and petrified wood; jasper,
smithsonite, and williamsite; and amazonite, petoskey stone, and
carnelian. (Two-thirds actual size.)]
[Illustration: These exquisite bowls, measuring 2 to 3 inches
across, are part of a set of 35 carved by George Ashley of Pala,
Calif., from gem materials found in the United States. Left to
right: paisley agate from California, petrified wood from Arizona,
black jade from Wyoming, chrysocolla from Arizona, and variscite
from Utah. (One-third actual size.)]
Shaping of gemstones is not limited to geometric faceting. Many stones,
especially those which are opaque or which produce stars and cat’s-eyes,
are cut as _cabochons_. This ancient, and probably oldest, cutting style
consists merely of a raised and rounded form. When extended completely
around the stone, the cabochon form results in a bead that can be
drilled and strung. Many cabochons, especially those of less expensive
gem materials, are now cut in large quantities to standard sizes in
order to fit mass-produced gem mountings.
Sculpting in gemstones is a much more intricate, nongeometric kind of
shaping. Although tools differ in detail, and the gem sculptor must
possess an artistic eye as well as lapidary skill, the basic processes
of sawing, grinding, and polishing are the same.
[Illustration: This coral carving, 11 inches tall without the stand,
owes its thin, graceful, willowy shape to the skill of the artist in
following the contour of a natural coral branch.]
[Illustration: The contemporary sculptor Oskar III J. W. Hansen
visualized and created the likeness of a spirited stallion in this
4½-inch turquoise carving, a gift of George Gilmer.]
[Illustration: This world-famed crystal ball, given to the
Collection as a memorial to W. R. Warner by his widow, represents
another phase of the lapidary art. Cut from a block of Burmese
quartz estimated to weigh 1000 pounds, this extremely valuable,
flawless, colorless sphere has a diameter of 12⅝ inches and weighs
106¾ pounds.]
4
GEM SUBSTITUTES
Because of their rarity and relatively high cost, the number of real
gems used throughout recorded times must be insignificant compared to
the number of gem substitutes used. There are records of glass and
ceramic imitations of gems as early as 3000 B.C. Certainly, the world
gem markets today are flooded with man-made gems. There even has been
developed a laboratory process for growing a coating of synthetic
emerald on the surface of a faceted stone of natural colorless beryl.
The recut gem looks like a natural emerald, and it has natural
inclusions that totally synthetic emeralds lack.
In general, gem substitutes can be classified as imitation stones,
assembled stones, reconstructed and altered stones, and synthetic
stones.
IMITATION STONES
Any material will serve as an imitation of a natural gem as long as it
resembles the real thing under casual examination. Because of the great
variety in types and colors available, glass and plastics are the most
commonly used materials for making imitation gems. Almost every gem has
been simulated effectively. The substitutes offer no difficulty of
identification to the expert, but many are deceptive to the layman.
ASSEMBLED STONES
It has been the practice for centuries to build up gemstones by fusing
or cementing a shaped piece of natural gemstone to another piece, or
other pieces, of inferior or artificial material.
A colorless common beryl crown cemented to a pavilion of green glass
produces an emerald doublet—part natural, part artificial—of good color
and high durability. A thin piece of beautifully colored opal cemented
to a base of inferior opal provides an assembled stone that looks like a
thick piece of high-quality opal. Triplets, and even stones in which
there are pockets of colored liquids or metal foil between the shaped
pieces, are known.
Usually, assembled stones are easily detected, since the joint will show
under magnification, but sometimes they are mounted in settings that
obscure the joint, and detection is more difficult.
[Illustration: Assembled imitation gemstones. If it were measured on
its natural ruby table, the assembled stone shown at top would have
all the characteristics of a large ruby, including refractive index.
The color of the quartz and glass combination (middle) depends on
the color of the liquid in the cavity. Since emerald is green beryl,
an inexpensive colorless beryl sandwich of green glass (bottom)
would appear to be an expensive emerald. The joints of assembled
stones often are hidden in the jewelry mountings.]
RECONSTRUCTED AND ALTERED STONES
Ruby fragments may be heated at high temperature to partially melt them
into a large mass that can be cut into a more valuable stone. Ruby is
the only stone that can be successfully reconstituted in this way, but
there are many other ways of tampering with natural stones to make them
more desirable.
Sometimes natural stones are backed with foil or a metallic coating to
enhance their color, to provide brilliance, or to produce a star effect.
It is said that in an inventory of the Russian crown jewels by the
Soviet Government, the ruby-colored Paul the First Diamond was
discovered to be a pale pink diamond backed by red foil. Today, some
diamonds are coated on the back with a blue film to improve their color.
Aquamarine, when pale greenish blue, may be heated in order to deepen
the blue color, and poorly colored amethyst may be heated to produce a
beautiful yellow-brown quartz, called citrine, that often is
misrepresented as topaz. By strong heating, the brown and reddish brown
colors of zircon can be changed to blue or colorless, both of which
states are unknown in natural zircon. Dyes, plastics, and oils are used
to impregnate porous gems such as turquoise and variscite, and even
jade. Off-color diamonds, when exposed to strong atomic radiation, can
be changed to attractive green, brown, and yellow colors, causing them
to resemble higher-priced _fancies_.
In the constant search for something new, gem suppliers sometimes
introduce into gemstones colors that are not always an improvement. For
example, the beautiful purple of some amethyst can be converted, by heat
treatment, to a peculiar green. Such an altered stone is marketed as
_greened amethyst_.
All of this tampering with gemstones complicates the problem of
identification, so it is a matter of serious concern to the gem trade.
SYNTHETIC STONES
For over 200 years mineralogists have been devising techniques for
producing synthetic minerals in the laboratory, and attempts have been
made, sometimes with considerable success, to apply these techniques to
the production of synthetic gemstones. To qualify as a synthetic
gemstone the man-made product must be identical chemically and
structurally with its natural counterpart. Sapphire, ruby, spinel,
emerald, and rutile in gem quality have been brought to commercial
production.
Two of the basic techniques used in producing synthetic gems are the
_flame-fusion_ and the _hydrothermal_ processes.
[Illustration: The Verneuil furnace, for making synthetic gem rough.
A mixture of hydrogen (H) and oxygen (O) burns almost explosively,
heating the fusion chamber (F) to high temperatures. For example,
powdered aluminum oxide and coloring agents are sifted down from
hopper (A) to the fusion chamber and form a cylindrical boule (B) on
an adjustable stand (C).]
In the flame-fusion process—invented in 1904 by the French chemist
Verneuil—powdered aluminum oxide, containing coloring agents, is sieved
down through the flame of a vertical blowtorch furnace. As it passes
through the flame, the powder melts and accumulates as drops on an
adjustable stand just below the flame, where it forms a single crystal
_boule_ of the synthetic rough. In a few hours a boule of several
hundred carats can be formed. When such furnaces are operated in banks
of several hundred units, the commercial production of corundum alone
becomes possible at the rate of many tons a year. Through the years, of
course, refinements have been made on Verneuil’s original furnace.
In the hydrothermal process, which differs greatly from Verneuil’s
flame-fusion process, crystals are grown from solutions of the raw
materials that have been subjected to varying conditions of very high
pressure and temperature. Some of the quartz used for electronics
purposes also is manufactured in this way.
Since chemical composition and crystal structure are the basic
characteristics by which a gemstone is identified, and these
characteristics are identical in both the manufactured stone and its
natural counterpart, the synthetic gemstones offer a very serious
challenge to those concerned with gem identification.
5
GEM LORE
All sorts of magic and symbolic properties have been ascribed to
gemstones through the ages; for example, the cat’s-eye has been
prescribed as a cure for paleness, citrine has been worn as a protection
from danger, and the opal cherished as the symbol of hope. The result
has been the creation of an intricate, chaotic, and contradictory but
interesting mass of gem lore.
Among the treasures in the Smithsonian’s Museum of Natural History is a
very old silver breastplate that once was in an ancient synagogue and
supposedly was modeled after the one worn by Aaron, the first high
priest of the Hebrews. In this plate are mounted twelve stones
representing the Twelve Tribes of Israel. Among Christians, the Twelve
Apostles also were represented symbolically by precious stones.
THE TWELVE TRIBES
Levi, _Garnet_
Zebulon, _Diamond_
Gad, _Amethyst_
Benjamin, _Jasper_
Simeon, _Chrysolite_
Issachar, _Sapphire_
Naphtali, _Agate_
Joseph, _Onyx_
Reuben, _Sard_
Judah, _Emerald_
Dan, _Topaz_
Asher, _Beryl_
THE TWELVE APOSTLES
Peter, _Jasper_
Andrew, _Sapphire_
James, _Chalcedony_
John, _Emerald_
Philip, _Sardonyx_
Bartholomew, _Sard_
Matthew, _Chrysolite_
Thomas, _Beryl_
James the Less, _Topaz_
Jude, _Chrysoprase_
Simon, _Hyacinth_
Judas, _Amethyst_
The number “12” seems to follow a chain of gemstone superstitions.
Gemstones were considered to have mystical relationship not only with
the Twelve Tribes and the Twelve Apostles but also with the Twelve
Angels, the Twelve Ranks of the Devil, and the Twelve Parts of the human
body.
Some stones were even endowed with astrological significance and were
believed to be in sympathy with the twelve zodiacal signs. On the basis
of an elaborate system of prognostications, an astrologer was considered
able to foretell future events by proper observance of changes in hue
and brilliance of the symbolic stones.
Aries the Ram, _Bloodstone_
Taurus the Bull, _Sapphire_
Gemini the Twins, _Agate_
Cancer the Crab, _Emerald_
Leo the Lion, _Onyx_
Virgo the Virgin, _Carnelian_
Libra the Scales, _Chrysolite_
Scorpio the Scorpion, _Aquamarine_
Sagittarius the Archer, _Topaz_
Capricornus the Goat, _Ruby_
Aquarius the Water Bearer, _Garnet_
Pisces the Fishes, _Amethyst_
Perhaps in our own space-oriented times the ancient superstitions
sympathetically relating certain gemstones with the planets will be
revived. In the distant past, moonstone, topaz, and other white stones
were believed to be in sympathy with the Moon, diamond and ruby with the
Sun, jasper and emerald with Mars, amethyst, topaz, and emerald with
Venus, carnelian, topaz, and amethyst with Jupiter, turquoise and
sapphire with Saturn, and rock crystal, agate, and emerald with Mercury.
Since Uranus, Neptune, and Pluto were unknown to the ancients, these
planets have not been represented by gemstones.
Of special interest to the American public are birthstones. Many
birthstone lists have been proposed, and in order to use this idea to
popularize gemstones the American jewelry industry has agreed upon an
official list. This list has served to bring about some uniformity in
the selection of birthstones for the twelve months.
January, _Garnet_
February, _Amethyst_
March, _Aquamarine_ or _Bloodstone_
April, _Diamond_
May, _Emerald_
June, _Moonstone_ or _Pearl_
July, _Ruby_
August, _Peridot_ or _Sardonyx_
September, _Sapphire_
October, Opal or _Tourmaline_
November, _Topaz_ or _Citrine_
December, _Turquoise_ or _Lapis lazuli_
All these associations and strange beliefs have served to create in the
general public a mental image of gemstones that gives to them an
increased exoticism and mysterious appeal far exceeding their monetary
value.
[Illustration: {zodiac symbols}]
6
PRINCIPAL GEM SPECIES
An excursion into the literature of gems would reveal that there is much
to be discovered about them other than the cold facts of gemology,
techniques of gem cutting, and tales of gem lore. When all the
information about an individual species is assembled, it provides a
sketch of a fascinating gemstone personality. Whole books have been
written about diamond—books filled with essays on its mining history,
natural occurrences, scientific significance, and best known cut stones.
In the following sections of this book, some of the facts about several
of the better known gem species have been gathered. The treatment is not
meant to be complete, but enough information is given so that the Museum
visitor may better understand and remember what he has seen.
For each species described there are color illustrations of certain
gemstones displayed in the collection. Several photographic and artistic
techniques have been used to emphasize the various aspects of the beauty
of these stones, many of which are the largest and finest of their kinds
known; however, not all of the finest gems are pictured here.
At the end of this descriptive section is a list of the significant
faceted gemstones in the collection. Obviously, this list will change,
because new gemstones constantly are being acquired.
DIAMOND
Diamond is the king of gems. It is a form of pure carbon, and it is the
hardest substance known; only diamond will cut diamond. It is
interesting that the humble graphite, its close relative, is also pure
carbon, but graphite is so soft that it is used as a lubricant and for
making the “lead” in pencils.
The ancients believed diamond to be indestructible, and even today many
people believe that diamond cannot be broken. Despite its great
hardness, however, diamond is not exceptionally tough, and it can be
split along what diamond cutters call its _grain_.
The diamond’s high brilliance results from its very high refraction, or
ability to bend light, and its fire is caused by its high dispersion, or
ability to divide light into its rainbow colors. However, only in
properly cut stones are diamond’s brilliance and fire developed to their
maximum.
At great depths in the crust of the earth and under conditions of very
high pressure and temperature, diamonds form in pipe-like bodies of
kimberlite, a heavy dark rock consisting primarily of two minerals,
pyroxene and olivine. In South Africa diamonds are mined from the
kimberlite, but they also are recovered there and elsewhere from beds of
sand and gravel where they have accumulated after being released from
their mother rock by erosion.
The world’s largest diamond deposits are in Africa, and names such as
Congo, Sierra Leone, and the Union of South Africa bring to mind
colorful legends of fabulous discoveries of diamond. Smaller deposits
are found in South America—in Brazil, British Guiana, and Venezuela—and
in Asia. Even in the United States some diamonds have been found.
India was the most important source of diamond until 1728, when
discoveries were made in Brazil. Among the important large diamonds
found in India were the Koh-i-noor, the Great Mogul, and, very likely,
the Hope Diamond. Like India, Brazil in turn declined as a major source
of diamond with the discovery and efficient recovery of large quantities
in South Africa.
[Illustration: The Hope Diamond, because of its long and dramatic
history and its rare deep-blue color, is probably the best known
diamond in the world. By speculation, the Hope is linked to the
famous “French Blue,” which was brought to France from India in 1668
to become part of the crown jewels of Louis XIV. The French Blue was
stolen in 1792 and never recovered, but in 1830 an extraordinary
44.5-carat blue diamond—presumably cut from the missing gem—came on
the market. It was purchased by Henry Thomas Hope of England and
became known by its present name. In 1949 the gem was acquired from
the estate of Mrs. Evalyn Walsh McLean by Harry Winston Inc., of New
York. Ten years later, Harry Winston, Inc., presented the gem (shown
here in actual size) to the Smithsonian Institution.]
Diamonds are extremely rare even in diamond mines. For example, the
famous South African mines contain only one part of diamond in more than
14 million parts of worthless rock. In spite of this, more than three
tons of gem- and industrial-quality diamond were mined in 1963.
Among the British crown jewels is a cut diamond weighing 530.20 carats
(more than 3¾ ounces), one of several stones that were cut from the
largest gem diamond ever discovered. The rough stone, known as the
Cullinan Diamond, weighed 3106 carats (almost 1¾ pounds) when it was
found at the Premier Mine in South Africa in 1905.
[Illustration: The Portuguese Diamond, weighing 127 carats, is the
13th largest cut diamond on record. More unusual, it is from Brazil,
and is thought to have been part of the Portuguese crown jewels. In
addition to its brilliant color flashes, it has a slight milky
fluorescence that causes it to “glow” even in artificial light.
(Actual size.)]
Diamonds vary from colorless to black and from transparent to opaque. As
they come from the mines, they are graded into two groups, gem and
industrial. Those whose color, imperfection, or shape make them useless
as gems—more than 8 out of every 10 carats mined—are used in industry.
Diamonds of industrial quality also are produced synthetically, and
these are used primarily in the manufacture of grinding wheels.
The best gem diamonds are flawless and are colorless or slightly blue.
Their value depends on their color, clarity, cut, and carat weight. Most
costly are those called fancies, which have a distinct color such as
blue, pink, green, or deep yellow.
PEARL
Pearl is included among gemstones only because it is a beautiful object
used as jewelry. As has been noted, pearl is not mineral because it is
formed by the action of a living organism. However, the pearl has long
occupied an important position among jewels, and it is unique in
requiring no lapidary art to enhance its beauty. Nature has perfected
pearls.
[Illustration: The strand of matched pearls was presented to
President Van Buren by the Imam of Muscat. The three baroque
(irregularly shaped) pearls are freshwater pearls from the Wabash
River in Indiana.]
The ancient Chinese believed that pearls originated in the brain of a
dragon. We now know, of course, that pearl is created by a secretion of
a mollusk. Very few mollusks have the ability to produce the fine
mother-of-pearl used in the jewelry trade, and even among those that
can, very few produce pearls with iridescence, or _orient_, as it is
known in the trade. Only two genera, the pearl oyster (_Margaritifera_)
and the pearl mussel (_Unio_) are important sources of the gem. Edible
oysters rarely produce pearls, and when they do, the pearls are of poor
quality.
The shells of pearl-producing mollusks are composed of layers of calcium
carbonate in the form of either calcite or aragonite. These layers,
cemented together with an organic substance known as conchiolin, are
known as nacre. The layer closest to the animal is deposited in tiny
overlapping patches, producing an iridescent effect caused by the
interference of light rays reflected from the plates making up the
nacre. The same material coats the surface of a gem pearl.
Seldom does a mollusk live out its time without attack by creatures
boring through its shell, or without intrusion through the normal shell
opening of tiny parasitic worms, sand, or other irritants. Usually inert
particles are forced against the inside of the shell, where they are
covered with layers of pearl that fasten them to the shell. This is the
source of most _blister pearls_. When the irritant remains in its fleshy
part, the mollusk deposits a protective shell of pearl to cover it
completely, and a spherical pearl may result. Pearls of less-symmetrical
shape, called _baroques_, are more common.
The value of a pearl depends on its shape, color, orient, and size.
Pearls of highest value are white with a faint tinge of pink or yellow,
possess fine orient, are round, and are free of surface blemishes. The
grading of pearls for color requires considerable experience to detect
delicate differences. Various classification names, such as “rosée” for
delicate pink shades, are used. Fancy colored pearls are those with a
strong yellow, bronze, pink, green, blue, or black color. Grading for
shapes, which differ markedly, is easier. Spherical pearls are usually
drilled for beads; pear-shaped or drop pearls are used in earrings and
pendants; and “boutons” or button-shaped pearls, with one flat side, are
used for ear ornaments, cuff links, and rings. Irregular, baroque pearls
and tiny seed pearls are used in jewelry designs with noble metals and
perhaps other gemstones.
The world’s finest pearls, called _oriental pearls_, come from the
fisheries of the Persian Gulf. Fine pearls also are found off the coasts
of Burma, Tahiti, New Guinea, Borneo, Venezuela and western South
America, and in the Gulf of California. Fresh-water pearls of high
quality, formed in pearl mussels, are found in various rivers in Europe
and the United States, especially in rivers in the Mississippi Valley.
A method of growing _cultured pearls_ has been well developed. A
mother-of-pearl bead is inserted in the oyster as an irritant, and the
animal is replaced in the sea in a cage. When oysters so treated are
recovered after a period of three to seven years, the beads in the
harvested crop usually are found to be coated with a layer of nacre up
to almost a sixteenth of an inch thick.
The cultured pearl can be identified only by the observance—through a
drill-hole or by X-ray—of the mother-of-pearl core, which had been
inserted in the oyster. An instrument called an endoscope, devised for
rapid testing of drilled pearls, relies on a beam of strong light
carried by a hollow needle. The needle is inserted into the drill hole,
and as it passes through the center portion of a natural pearl a flash
of light, reflected through a mirror system in the needle, is observed.
CORUNDUM
(RUBY AND SAPPHIRE)
Both _ruby_ and _sapphire_, which are second only to diamond in
hardness, are of the mineral species corundum, an oxide of aluminum.
They are identical in all characteristics except color. Most corundum is
opaque, and it is mined in large quantities for use as an abrasive. In a
few places, such as Moguk in Upper Burma and in Ceylon, clear corundum
is found that is suitable for use as a gem.
Red corundum is known as ruby. Its color, caused by traces of chromium,
ranges from rose through carmine to a dark purplish red referred to as
pigeon’s blood red. Rubies of this very desirable latter color often are
called Burma rubies, and they are the most costly of all the corundum
gems.
All gem corundum having a color other than red is sapphire. The name
sapphire means blue, and this is the color most frequently associated
with this gemstone. The finest sapphires are a velvety cornflower blue,
and they come from Kashmir. Blue, white, yellow, gold, pink, and all the
other colors of corundum are caused by the presence of slight traces of
iron, chromium, titanium, and other metals present as dissolved
impurities in the aluminum oxide. Frequently sapphires are found that
show patches of blue and yellow, or that have alternating zones of red
and blue. Pure corundum is colorless.
[Illustration: A piece of uncut ruby, from Burma, and five small
rubies of about half a carat each, from Ceylon. All have the classic
“pigeon’s blood” color. (Actual size.)]
Most gem corundum comes from the Orient, at localities such as Moguk in
Upper Burma, near Bangkok in Thailand, Kashmir in India, and Ceylon.
Because of this primarily Asian origin, the word _oriental_ often is
used with the names of other gems to denote a sapphire of a particular
color. For example, green sapphire sometimes is called oriental emerald,
and the yellow sapphire sometimes is called oriental topaz.
[Illustration: The sapphires in this group vary in color from deep
blue to gold, and they come from widely separated localities. The
scatter of small multicolored stones came from Montana, and the
magnificent 93-carat golden sapphire, encircled by the gold
bracelet, came from Burma. (Slightly less than half actual size.)]
There are some notable exceptions to the generally oriental occurrence
of corundum. Some good-quality ruby has been found in North Carolina,
and sapphire of many colors has come from Montana.
During the formation of a corundum crystal, extremely small needle-like
inclusions of rutile sometimes occur in the hexagonal pattern of the
host crystal. When such inclusions are arranged in this way by nature,
they cause, in properly cut stones, internal reflections that produce
the optical phenomenon known as asterism. The effect is that of a
6-rayed star, and the gems in which asterism occurs are known as star
sapphires and star rubies. Asterism is rarer in ruby.
[Illustration: The Star of Asia, weighing 330 carats, is one of the
finest star sapphires in the world. It is of a clear, deep blue
color and has a strong, sharply defined, 6-rayed star. (Actual
size.)]
[Illustration: Cutting a star stone requires careful attention to
the directions in which the cuts are to be made. Failure to align
the stone properly with the axis of the crystal will produce a stone
with an off-center, crooked, or dim star, or may even eliminate the
star completely.]
CRYSTAL AXIS
POSITION STONE MUST TAKE TO SHOW STAR
OTHER STAR STONES MAY BE CUT, BUT MUST BE IN THE SAME POSITION WITHIN
THE CRYSTAL
ROUGH SAPPHIRE CRYSTAL
CRYSTAL AXIS
Since corundum is easily manufactured, synthetic ruby and sapphire are
used extensively in jewelry. The synthetic stones can be distinguished
from natural stones by microscopic examination of the kinds of
inclusions and internal defects.
VARIETIES
Ruby: Red.
Sapphire: Blue, yellow, pink, green, colorless, and any color except
red.
Star sapphire: Colored as sapphire and showing asterism.
Star ruby: Red and showing asterism.
BERYL
(INCLUDES EMERALD AND AQUAMARINE)
Beryl is probably the most widely used colored gemstone, and under its
several names in the gem world it is probably the best known. When it is
a rich green it is known as _emerald_, and when it is the blue-green of
sea water it is called _aquamarine_. Varieties such as the rose-pink
_morganite_, golden-yellow _heliodor_, and colorless _goshenite_ are
less well known than emerald and aquamarine but are equally attractive
and satisfactory gemstones.
Beryl is beryllium aluminum silicate. It frequently occurs in
well-formed hexagonal crystals, and its many colors result from the
presence of very small percentages of several different elements.
Emerald owes its rich green color to traces of chromium, and the
detection of this element is one of the means of identifying true
emerald. Aquamarine, comprising the green and blue-green beryls, gets
its color mainly from traces of iron. Practically all of the deep blue
aquamarine available in jewelry stores results from the heat treating of
greenish beryl or certain yellow-brown beryls. The stones are heated
carefully to about 800° F., and the color change is permanent. The
element lithium accounts for the color of pink beryl. As with
aquamarine, the color of yellow beryl is now considered to be the result
of traces of iron rather than uranium, as previously thought. Pure beryl
is colorless.
Beryl usually is found in pegmatites, which are very coarse-grained
granite rocks formed by the cooling of molten material far beneath the
earth’s surface. As the rock cools and beryl and other crystals are
formed, the stresses introduced are so great that the crystals
frequently shatter so badly they are useless as gem material.
Frequently, too, impurities are introduced during crystal formation, and
consequently the gem materials are found only where the crystals were
able to form without interference—such as in openings or cavities in the
rock.
Tremendous beryl crystals weighing as much as several tons, but not of
gem quality, have been discovered in a few localities. Large crystals of
gem quality also occur in nature, and large cut stones of aquamarine and
other colors of beryl are relatively common. Among the fine examples of
beryl in the National Gem Collection is a remarkably large (2054-carat),
flawless cut stone of rich yellow-green. This gem and others in the
collection weighing 1363 carats, 1000 carats, 914 carats, and 578 carats
accentuate the occurrence of large gem crystals of beryl in Brazil.
[Illustration: Four large cut stones, all from Brazil, illustrate
the color range of beryl. Top, a 578-carat green beryl; left, a
235-carat morganite, gift of Mr. and Mrs. Frank Ix, Jr.; bottom, a
133-carat gold beryl; and, right, a 187-carat aquamarine. (Half
actual size.)]
The finest emeralds are not found in pegmatites. At Muzo in Colombia,
the most prolific source of the finest emeralds, they occur in veins
with calcite, quartz, dolomite, and pyrite. The veins cut through
dark-colored, carbonaceous limestone and shale. Mining at Muzo began 350
years ago and still continues sporadically to meet market requirements.
Russian emeralds occur as good-sized crystals in mica schist, a
metamorphic rock. They occur there with chrysoberyl, phenakite, and
common beryl. Some of the smaller stones have good color and have been
cut into valuable gems. Brazil, which produces many extraordinary
aquamarines and other beryls, has not produced quality emeralds.
Periodically, over the centuries, there have been reports of new
discoveries of emerald, but so far none of these has begun to rival the
Muzo source in either quantity or quality of the gems produced.
[Illustration: This tremendous golden beryl from Brazil, weighing
2054 carats, is the largest cut beryl known of this color. Cut
stones of this size that contain no visible flaws or inclusions are
most unusual. (Three-fifths actual size.)]
Although Brazil supplies the finest aquamarine and Colombia the finest
emerald, several localities in the United States are sources of
good-quality beryl of these colors. Foremost among these localities are
Maine, California, and Connecticut for aquamarine and North Carolina for
emerald. Morganite of pale pink to deep peach color, from California, is
also notable. Various New England mines in Maine, New Hampshire, and
Connecticut and the gem mines of the Pala and Mesa Grande districts of
California have produced other colors of gem beryl. However, most of the
beryl mined in the United States is used as an ore for beryllium, as
little of it is of gem quality.
Because of its hardness (about 8), vitreous luster, beautiful color, and
rarity, emerald always has been highly prized as a gem. Fine-quality
emeralds may be more costly than fine diamonds. Other kinds of beryl
have the same physical properties as emerald, but since they are less
rare their relative value is lower.
Synthetic emerald of high gem quality has been marketed successfully. A
synthetic substitute for aquamarine is also available; it is really a
synthetic blue spinel.
VARIETIES
Emerald: Grass green
Aquamarine: Blue green
Morganite: Pink
Heliodor: Yellow
Goshenite: Colorless
TOPAZ
[Illustration: Three different cutting styles and colors of topaz.
From top, a 235-carat colorless stone from Colorado, a 171-carat
dark champagne-colored stone from Madagascar, and a 129-carat
sherry-colored stone from Brazil. (Slightly less than actual size.)]
Because yellow is the most popular color of topaz it has become
customary to believe that all topaz is yellow. Also, there is a tendency
to believe that all yellow gemstones are topaz. Neither belief is
correct. Stones of yellow, sherry, blue, pink, and colorless topaz all
make beautiful gems, and their characteristics are identical except for
color. On the other hand, citrine (a yellow quartz), although entirely
unrelated to topaz, often is disguised in the trade under the names
Brazilian topaz, topaz quartz, or just topaz. Great numbers of stones
described and sold as yellow topaz really are the much commoner citrine,
which has few of the characteristics of fine topaz.
[Illustration: A cushion-cut topaz from Brazil that weighs 1469
carats. It is an odd shade of yellow-green.]
[Illustration: A 3273-carat topaz of soft blue that came from
Brazil. The Smithsonian Institution had this unique gem cut by Capt.
John Sinkankas of California. For several years it was the largest
topaz in the collection. (Both gems are shown in actual size.)]
Topaz, an aluminum fluosilicate, has a hardness of 8, a vitreous luster,
and a relatively high refractive index. It is found in near-perfect
crystals that range in size from very small to very large, with some
giants weighing as much as several hundred pounds. Most of these
crystals, especially the largest ones, are colorless, a characteristic
that indicates relatively high purity of composition. Although topaz
gems have little fire, they take a high polish and can be very
brilliant. Great care must be taken in cutting and polishing topaz
because of its ready cleavage. The desired cut and high polish can be
secured by avoiding excessive heat or pressure during the operation and
by planning facets so that none lies exactly parallel to the cleavage
direction.
Although crystals of gem-quality topaz are found in many localities,
perhaps the splendid blue ones from Russia and the yellow, wine, blue,
and colorless ones from Brazil are best known. Some fine topaz has been
found in the United States in such widely separated areas as New
Hampshire, Texas, Colorado, and California. The light, golden brown
topaz from Colorado has an unfortunate tendency to fade in strong
sunlight. It remains to be seen whether similar topaz coming recently
from comparable occurrences in Mexico also will fade. By a system of
heating and cooling, certain of the red-brown topaz crystals from Ouro
Preto, Brazil, can be converted to colors ranging from salmon pink to
purple red. Quick heating to high temperatures can completely remove
color, and sudden or uneven cooling may cloud or crack the stone.
OPAL
Opal has been admired for its great beauty since ancient times, but this
gemstone lacked commercial appeal until the discovery of the Australian
black opal late in the 19th century.
Opal is somewhat brittle, is sensitive to heat, and, in some cases,
tends to deteriorate despite the best of care. Therefore, this stone
lacks many of the physical characteristics required for an ideal gem.
These deficiencies would eliminate other species from the list of
gemstones, but the great beauty of its flashing and shifting color
patterns has made opal increasingly popular. Even its name, coming from
the ancient Sanskrit “upala,” means precious stone.
With a hardness between 5½ and 6½, opal is the softest of the more
popular gems. It is sufficiently hard, however, to be used in jewelry,
where its setting usually helps to protect it from shock and abrasion.
[Illustration: Black opal, so called because the color flashes
appear against a dark background, is found in Australia. It is quite
rare, and large pieces such as the ones shown here have become
extremely valuable. (Almost actual size.)]
Opal is unlike most gemstones in that its flashing color is not due to
the color of the stone itself, or even to the color of its included
impurities. Rather, it is due to the way in which tiny opal particles
are grouped during its formation. Detailed photographs taken through an
electron microscope show clearly how precious opal is deposited as
spheres that are so small they are indistinguishable under powerful
optical microscopes. These spheres are packed together in very orderly
networks, row upon row and layer upon layer, with tiny open spaces, also
in rows, between them. Masses of common opal lack this orderly internal
arrangement of spheres. White light striking the precious opal is
reflected independently by each row of spheres, much like the
reflections from a series of slats in a venetian blind. Since these rows
of spheres are spaced at distances approximately the same as the
wavelength of light, a phenomenon known as _diffraction_ occurs. The
separate reflections interfere with each other in an organized manner,
cancelling out some of the light wavelengths and reinforcing others,
producing color. The brilliant color flashes are of different hues
depending on the sizes of the spheres of opal and, therefore, the
distances between rows. To provide the best display of this optical
effect, opal is almost always cut in cabochon form rather than as
faceted stones.
[Illustration: Fire opals have rich fire; some have background
colors that vary from bright yellow through orange and red; and some
are colorless. Stones such as the ones shown here, which weigh 7,
11, and 22 carats, have made Querétaro, Mexico, famous as their
source. (Actual size.)]
[Illustration: This rare 34-carat opal from Brazil resembles closely
the opals found in Australia. (Actual size.)]
Common opal, which shows milky opalescence, does not exhibit color
flashes, and it is not used as a gemstone. Each of the common
varieties—such as hyalite, cacholong, and hydrophane—has its own
slightly different set of characteristics, but only precious opal, with
its dazzling color display, is important for gem purposes. To take full
advantage of the small amounts of gem material available, or to bring
out its color better, _precious_ opal is often cut as thin pieces and
mounted as doublets on some other backing. Also, the seams in rock
sometimes are cut so that the thin layer is exposed on a thicker backing
of the adjoining rock. Precious opal, or gem opal, is classified as
_white opal_ when the color flashes are in a whitish or light
background, _black opal_ when the background material is gray,
blue-gray, or black, and _fire opal_ when the background is more
translucent and red, reddish orange, or reddish yellow.
Precious opal has been found in several areas of the world—in nodules,
in seams in rock, or as replacements of other minerals or even of wood
and shell. Hungarian deposits were well known in Roman times, but these
and other deposits became insignificant with the discovery of opal in
Australia in the late 19th century. Opal deposits were discovered in
1889 at White Cliffs in New South Wales, and other important discoveries
in Australia followed, including deposits at Lightning Ridge in New
South Wales that produce very dark stones and the rich fields of white
opal at Coober Pedy in South Australia. Mexico has remained for a long
time the principal source of richly colored fire opals, with the most
important deposits located in the state of Querétaro, where mines have
been worked intermittently since 1835. This has made the town of
Querétaro today the center for the trade and cutting of Mexican opal.
VARIETIES
White opal: Color flashes in light-colored background material
Black opal: Color flashes in dark gray or bluish background material
Fire opal: Orange or reddish background material
SPINEL
Two of the more famous stones in the British crown jewels are the Black
Prince’s Ruby and the Timur Ruby, but neither of these stones is really
ruby. Like the great red gem in the crown that belonged to the Russian
Empress Catherine II, these two British stones are spinel. Although
spinel occurs in many colors, such as yellow, green, violet, brown, and
black, it is the red spinel that usually is seen in the gem trade. There
are several varieties of red spinel, such as _ruby spinel_, _balas
ruby_, _rubicelle_, and _almandine spinel_—all of which refer to the
color resemblance to ruby.
[Illustration: The hues and tints of spinel show subtle variations
that are matched only by those of tourmaline. Unlike tourmaline,
however, spinel may be bright ruby red. The cut stones curving
around two pieces of rough from Burma weigh (left to right) 30
carats (Ceylon), 34 carats (Burma), 36 carats (Burma), 30 carats
(Ceylon), and 22 carats (Ceylon). (Three-fourths actual size.)]
Spinel is an oxide of magnesium and aluminum, and it is not related to
ruby. However, because its hardness (8) is only slightly less than that
of ruby and its brilliance is about equal to that of ruby, spinel makes
an excellent substitute for that gem. Also, because it is more
plentiful, spinel costs much less. It is interesting that red spinel,
like ruby, gets its color from the presence of traces of chromium.
Synthetic blue spinel is widely used as a substitute for aquamarine, and
synthetic spinels of other colors are used as substitutes for many gems.
However, the synthetic stones are not ordinarily made in the subtle
shades so characteristic of natural spinel. Completely colorless spinel
apparently exists only as a synthetic material. Actually, because of its
hardness, durability, and many attractive colors, spinel makes a fine
gemstone in its own right.
Like ruby and several other gemstones, spinel is found chiefly in the
gem gravels of Ceylon, Burma, and Thailand. Appreciable amounts of
spinel occur in the Ceylon gem gravels as worn, rounded pebbles of many
colors. In the Burmese gravel deposits the spinel is often found as
well-formed octahedral crystals. Near Moguk, in Burma, spinel has been
found in its original position in the limestone rocks as well as in the
eroded stream deposits.
VARIETIES
Almandine spinel: Purplish red
Rubicelle: Orange-red
Balas ruby: Rose red
Ruby spinel: Deep red
Chlorospinel: Translucent grass green
Ceylonite or pleonaste: Opaque dark green, brown, or black
Picotite or chrome spinel: Translucent dark yellow-brown or
green-brown
QUARTZ
(INCLUDES ROCK CRYSTAL, AMETHYST, AND CITRINE)
Few gemstones can compete with quartz for variety of color. Having a
hardness of 7 and occurring in many beautiful varieties, only the
relative abundance of quartz prevents the species from attaining top
rank among gemstones.
The two kinds of quartz, crystalline and cryptocrystalline
(fine-grained) quartz, occur in all kinds of mineral deposits throughout
the world. Much of this material is suitable for cutting gems.
Colorless crystalline quartz, or _rock crystal_, makes attractive
faceted gems, and it is used as a suitable substitute for diamond and
zircon even though it lacks the fire and brilliance of those gemstones.
Some very large, flawless crystals of colorless crystalline quartz have
been found. The great Warner Crystal Ball, with a diameter of 12⅞ inches
and weighing 106¾ pounds, was cut from such a crystal. In addition to
the name rock crystal, colorless crystalline quartz appears in the
jewelry trade under such names as rhinestone (not to be confused with
the glass substitute), Herkimer diamond (from Herkimer County, N. Y.),
and Cape May diamond (from Cape May, N. J.).
The most popular variety of quartz is _amethyst_, a transparent form
whose color ranges from pale violet to deep purple. In many cut stones
of amethyst the color intensity changes sharply from section to section.
This is due to irregular color zoning common to amethyst crystals. The
actual cause of the purple color in amethyst is not very well
understood. There are fewer cut stones of amethyst in very large sizes
because of the rarity of large, flawless, well-colored crystals.
[Illustration: This 4500-carat pale smoky quartz egg from California
rests on a gold stand set with Montana sapphires. The unique gem was
cut and its stand was designed and made by Capt. John Sinkankas as a
difficult exercise in the lapidary art. The quartz egg is 4 inches
long and almost 3 inches in diameter.]
The name _citrine_ (from the French word for lemon) attempts to describe
the yellow color of another variety of quartz. Actually, the normal
coloring of citrine varies from yellow to red-orange and red-brown, but
the yellow sometimes rivals the yellow of topaz. In addition to the
normal color range, the colors of citrine may grade through a grayish
yellow variety known as _cairngorm_ and a grayish variety called _smoky
quartz_ to a black variety called _morion_. Other varieties that add
color dimensions to the group of quartz gemstones are _rose quartz_ and
_milky quartz_. Like amethyst, the reason for the color in rose quartz
has not been definitely established. Milky quartz owes its color to
myriads of tiny cavities containing water or liquid carbon dioxide.
[Illustration: A 783-carat step-cut citrine of deep, rich color
dwarfs a 278-carat brilliant-cut citrine (at left), a 90-carat smoky
quartz, and a 91-carat briolette of citrine. The smoky quartz, from
Switzerland, is so dark that it appears to be opaque. The other
three stones came from Brazil. The briolette and brilliant-cut
citrines were cut and donated to the Smithsonian Institution by
Albert R. Cutter. (Slightly less than half actual size.)]
The range of color in quartz is somewhat surprising, considering that
the mineral is a simple silicon dioxide. Some of the colors, as with
corundum and some other gemstones, are due to traces of impurities. In
quartz, these consist mainly of oxides of iron, manganese, and titanium.
However, all the reasons for quartz coloration in its many varieties are
not known.
[Illustration: Pastel rose quartz has a delicate beauty in any cut.
The 375-carat step cut (top), the 84-carat step cut, and the
46-carat marquise came from Brazil. (Two-thirds actual size.)]
[Illustration: Amethyst, a purplish quartz, is the birthstone for
February. Here it is represented by a 1362-carat stone from Brazil
(top), a 54-carat stone from Pennsylvania (left), and a 21-carat
stone from North Carolina. (Almost actual size.)]
In addition to possessing wide variation of color, quartz, like sapphire
and certain other gemstones, can exhibit asterism or chatoyancy. The
well-known _tiger’s-eye_ from West Griqualand, South Africa, owes its
eye effect to the fact that its material is a replacement of fibrous
asbestos by cryptocrystalline quartz. The color of tiger’s-eye arises
from the partial alteration of the asbestos to yellow-brown iron oxides
before it is replaced by quartz. Inclusions of rutile, tourmaline, or
actinolite needles may produce attractive patterns in quartz, but they
do not always cause chatoyancy. The material containing such inclusions
is called sagenitic quartz, or it may be descriptively named, such as
rutilated quartz, tourmalinated quartz, and so forth. Sagenitic quartz
is usually cut as cabochons rather than as faceted stones since the
inclusions are of greater interest than the quartz itself.
If the foreign inclusions consist of tiny flakes of hematite or mica,
the quartz assumes a spangled appearance and is called _aventurine_.
Crystals of quartz varieties that are opaque or that contain visible
inclusions normally are cut as cabochons to take advantage of the body
color or to make the inclusions more visible. Crystals of the
transparent varieties are fashioned in any of several cutting styles,
depending on whether it is desired to take maximum advantage of color or
of brilliance. Because of its availability in fairly large, flawless
pieces in various colors, quartz has been used extensively in carving.
The Chinese have excelled in carving large, ornate objects of rock
crystal.
Although quartz occurs in many varieties and its crystals are cut in
many styles, it is easily identified by its refractive index of 1.55,
specific gravity of 2.65, and hardness of 7.
CRYSTALLINE VARIETIES
Amethyst: Purple to violet
Cairngorm: Smoky yellow
Citrine: Yellow to red-orange and red-brown
Milky quartz: White
Morion: Black
Rock crystal: Colorless
Rose quartz: Rose to pink
Smoky quartz: Gray to black
CRYPTOCRYSTALLINE VARIETIES (CHALCEDONY)
Agate: Pronounced color banding
Aventurine: Inclusions of sparkling flakes
Bloodstone: Dark green dotted with red
Carnelian: Red to yellow-red
Cat’s-eye: Chatoyant
Chrysoprase: Green
Jasper: Opaque brown to red-brown, green, yellow, etc.
Onyx: Color banding in straight layers of contrasting color
Sard: Light to dark brown
Sardonyx: Sard or carnelian bands alternating with white bands
Tiger’s-eye: Bright brownish yellow, sometimes blue: chatoyant
CHRYSOBERYL
(INCLUDES ALEXANDRITE AND CAT’S-EYE)
With color ranging from shades of yellow and brown through blue-green to
olive, and with a hardness of 8½, chrysoberyl has most of the
characteristics necessary for a fine gem. Rare stones of high-quality
chrysoberyl demand fairly high prices, and they are sought eagerly by
the connoisseur of gemstones.
Chrysoberyl is beryllium aluminate, and thus is closely related to the
gemstone spinel, which is magnesium aluminate. When pure, chrysoberyl is
colorless and relatively uninteresting as a gemstone because of its lack
of color dispersion and its moderate refractive index of 1.75. However,
few pure samples are known, as chrysoberyl normally contains some iron
or chromium in place of aluminum and some iron in place of beryllium. As
a result of such impurities, the color of chrysoberyl my be yellowish,
greenish, or brownish.
Chrysoberyl and beryl are the only important gemstones containing the
element beryllium. The minerals beryllonite, euclase, hambergite, and
phenakite also contain this element, but they are rare and seldom are
seen as cut gems.
[Illustration: One of the finest chrysoberyl cat’s-eyes in existence
is the 58-carat Maharani from Ceylon. (Actual size.)]
The _alexandrite_ variety of chrysoberyl has two colors in delicate
balance, and it changes from a columbine red to an emerald green when
viewed under different light. When viewed in daylight, which is richer
in green, the color balance shifts toward green, and that hue is seen by
the observer. Under artificial light, normally richer in red, the color
balance shifts toward red, and the stone seems to have changed to that
color. This extremely rare stone, named after Czar Alexander II of
Russia, is found only occasionally, in Russia and Ceylon. The Russian
stones, found with emerald in mica schist, tend to be smaller than the
Ceylon stones and have a color change going from emerald green to
violet-red. The Ceylon stones, found as pebbles in gem gravels, have a
color change going from a less-emerald green to a browner red. The
66-carat, record-size alexandrite in the National Collection shows the
color change typical of Ceylon stones. A synthetic stone is commonly
marketed as synthetic alexandrite, but this substitute not only is
man-made but is actually synthetic corundum instead of synthetic
chrysoberyl.
[Illustration: In addition to its fine cat’s-eyes and its
color-changing alexandrite varieties, chrysoberyl occurs in handsome
stones that vary in depth of color. Shown here with an uncut twinned
crystal of gem quality from Brazil are a 46-carat stone from Brazil
(left) and a 121-carat stone from Ceylon. The uncut crystal is a
gift of Bernard T. Rocca, Sr. (Two-thirds actual size.)]
_Cat’s-eye_ chrysoberyl contains myriads of tiny fiberlike channels
arranged in parallel position. When the stone is cut as a cabochon, a
band of light is reflected from the curved top of the stone, producing
an effect that resembles the slit pupil of a cat’s eye.
VARIETIES
Alexandrite: Green in daylight, changing to red in artificial light
Cat’s-eye: Chatoyant
TOURMALINE
Because of its great color range, which includes pink, green, blue,
yellow, brown, and black in many different shades and combinations of
shades, tourmaline is one of the most popular of the colored gemstones.
Tourmaline with a color near emerald green is particularly popular.
Chemically, tourmaline is a very complex borosilicate, and its color is
determined by the various elements present in it. Tourmaline crystals
having sodium, lithium, or potassium are either colorless, red, or
green; those having iron are blue, blue-green, or black; and those
having magnesium are colorless, yellow-brown, or blackish brown.
Some crystals of tourmaline are of two colors, and stones of mixed
colors, such as pink and green, can be cut from these. The color mixing
may show as zoning with the core color of the crystal overlaid by
another color and perhaps even additional layers of other colors. Zoned
crystals with a core of deep pink covered by a layer of green have been
called “watermelon tourmaline.” Because its refractive index of about
1.6 is too low to give it marked brilliance, and its color dispersion is
too low to give it fire, the tourmaline relies almost solely on the
beauty of its color for its rank in popularity.
Although tourmaline has a low refractive index and low dispersion, it
exhibits remarkable dichroism. In other words, it can present different
tints to the viewer depending on the direction that the light is
traveling through the crystal. When viewed down the long, or vertical,
axis of the crystal, the color of tourmaline is much stronger than when
viewed from the side. This means that if the crystal is dark the cutter
will have to cut the stone with the flat part, or table, parallel to the
long axis of the crystal. The color of the gemstone then will be
lightened when viewed from its table, since this is the direction of
lighter color. Similarly, the table of a lighter colored crystal can be
cut perpendicular to the long axis in order to produce a deeper colored
gem.
[Illustration: Green seems to be the best known commercial color of
tourmaline, but this extremely variable gem species exhibits many
subtle shades of color, as shown here. At upper left, a 104-carat
stone from Mozambique; at upper right, a 173-carat stone from
Mozambique; at lower left, a 111-carat stone from Manchuria; and a
35-carat stone from Brazil. (Actual size.)]
Some tourmaline crystals contain threadlike tubes or inclusions of
microscopic size running parallel to its length. When cut as cabochons,
such crystals give a good “cat’s-eye” effect.
Tourmaline has no distinct cleavage and has a hardness somewhat above 7,
and these characteristics make the stone sufficiently resistant to
normal shock and wear so that it is highly satisfactory for use in
jewelry.
Noted deposits of tourmaline are located in the Ural Mountains of
Russia, Ceylon, Burma, South-West Africa, Madagascar, Brazil, Maine, and
California. Crystals from each of these localities seem to have their
own color specialties. The deposits in San Diego County, Calif., are
unique in that all colors except brown are found there. In the early
1900’s pink and red tourmaline was shipped from there to China for
carving, but this thriving trade stopped with the end of Chinese
imperial reign. The tourmaline deposits at Paris, Auburn, and Hebron,
Maine, have furnished a number of excellent gems, especially of blue and
green colors.
VARIETIES
Achroite: Colorless
Indicolite: Blue
Dravite: Brown
Schorl: Black
Rubellite: Pink
ZIRCON
Zircon, because of its high refractive index and high dispersion,
approaches diamond in degree of brilliance and fire. On only casual
examination it is quite possible to mistake a well-cut, colorless zircon
for a diamond. However, a careful examination of the back facets of such
a stone, when viewed through the table, would show strong double
refraction, a characteristic of zircon but not of diamond. Zircon’s
double refraction makes the back facet edges appear doubled. Since
diamond is “singly refracting,” it cannot produce this double appearance
of the back facets.
Zircon is brittle and has a hardness of just over 7, while diamond’s
hardness, as we have seen, is rated at 10. After being worn in jewelry
for a long period of time, zircon will show signs of chipping on the
facet edges. Under the same conditions, diamond would remain unchanged.
Because of this tendency for facet edges to chip, it is the practice in
the gem trade to pack cut zircons separately. If a number of zircons
were placed in the same paper packet there would be a risk of “paper
wear.”
In the gem trade, the most important zircons are those that are
colorless, golden brown, or sky blue. Such stones originally were
reddish brown zircon pebbles from Indochina, but they have been
converted by being subjected to temperatures approaching 1800° F. for
periods of up to two hours. When the original zircons are heated in a
closed container, the stones become blue or colorless; when a flow of
air is allowed through the container, the stones become golden yellow,
red, or colorless. In most of these converted stones the color remains
quite stable, but in some it may revert to an unattractive greenish or
brownish blue after a period of time.
[Illustration: The beautiful colors of these brilliant zircons are
the result of heat treatment given to natural, reddish brown stream
pebbles. The three stones at the left (from top) weigh 118, 103, and
98 carats, and the ones on the right weigh 106 and 29 carats. The
106-carat stone came from Thailand, the others from Indochina.
(Four-fifths actual size.)]
In addition to being reddish brown, natural zircon may vary from almost
colorless to yellow, red, orange, and brown or from yellow-green to dark
green and, occasionally, blue.
The most important producing areas of gem zircon are in a region of
Indochina that comprises parts of Thailand, Viet Nam, and Laos.
Additional gem zircon, like so many of the other gem species, is
recovered from near Moguk in Upper Burma and from the gem gravels of
Ceylon.
There is no synthetic zircon on the market, but a bright blue synthetic
spinel is sometimes used to simulate zircon successfully.
PERIDOT
The relative rarity of peridot and the ease with which it can be
simulated in glass, whose luster it approximates, probably account for
the low popular demand for this gemstone. Although peridot has little
brilliance and no fire, its unusual color and glassy luster produce a
unique effect that serves to make it attractive.
The color of peridot is an unusual bottle green that shades, in some
stones, toward yellow-green and, more rarely, toward brown. In 1952 it
was discovered that almost all of the brown gems believed to have been
peridot in various gem collections were actually of an entirely
unrelated species, which since has been named sinhalite. Brown peridot
still remains rare and is somewhat of a collector’s item.
[Illustration: To exhibit its unique color to best advantage,
peridot usually is cut so as to have a relatively large table, as
shown in these examples. The largest gem, weighing 310 carats, is
from the Egyptian island of Zebirget in the Red Sea and is the
largest cut peridot known. The other two, weighing 287 carats and
109 carats, are from Burma. (Three-fifths actual size.)]
The green of peridot, which is quite different from the green of other
gemstones, is due to some iron included in its composition. It is
suspected that a trace of nickel contributes to the liveliness of the
color.
[Illustration: This photo shows the color of peridot projected onto
the background. The larger gem is the 310-carat stone shown in the
prior illustration. The stone on the right weighs 109 carats and is
from Burma; the other peridot weighs 46 carats and is from Egypt.
(Almost actual size.)]
Peridot has a hardness of only 6½ and a rather strong tendency to
cleave, and these characteristics reduce its value for use in jewelry
exposed to rough wear. It is better used in pins, earrings, and pendants
than in rings.
Peridot is a gem name for the common mineral olivine, a magnesium
silicate. Olivine is fund in numerous places, and small gemmy pieces are
found in many localities. Many of the largest and best gems of peridot
have come from mines on the Egyptian island of Zebirget (Island of St.
John) in the Red Sea, but most gem peridot now comes from Burma. Great
numbers of small stones have been cut from olivine found in Arizona
gravels.
Centuries ago, peridot was known by the name topaz, since the stones
came from Topazos, the island now known as Zebirget. The name topaz, as
we have seen, is used today for an entirely different mineral species.
SPODUMENE
Spodumene, a lithium aluminum silicate, is one of the very few gemstones
containing lithium. It has had more importance as a gemstone in the
United States than elsewhere, a situation due to early discoveries of
unique occurrences of a lavender-pink variety at Branchville, Conn., in
1879 and in San Diego County, Calif, about 20 years later. At the time
of the discovery of the California material, the variety was named
_kunzite_ in honor of G. F. Kunz, a noted American gemologist of the
times.
[Illustration: The 177-carat kunzite (at lower left) is a large
flawless stone cut from California material of this variety of
spodumene. It was given to the Smithsonian Institution by the
American Gem Society. The other stones, all from Brazil, represent
the more usual shades of spodumene. They weigh 327 carats (top
left), 256 carats (top right), and 69 carats. (About half actual
size.)]
The finding of a bright green variety, _hiddenite_, in North Carolina
about 1880 greatly stimulated the interest of American gem collectors.
Some of the bright green spodumene coming from Brazil in recent years
compares very favorably in color with North Carolina hiddenite. Other
than in a scattered few of these unusual occurrences of kunzite and
hiddenite, spodumene usually is found in yellow and yellow-green shades,
with Brazil and Madagascar being the chief sources.
[Illustration: This 880-carat kunzite from Brazil is one of the
largest stones of its kind. (About actual size.)]
Spodumene has a hardness of about 7, but with a refractive index of
about 1.66 and a low dispersion there seems to be relatively little to
recommend it as a gemstone. The fact that it exhibits a very strong
tendency to cleave in two different directions would seem to rule it out
completely as being too difficult to cut. Nevertheless, the production
and purchase of cut stones of spodumene persist because of the beauty of
the gem.
The kunzite and hiddenite varieties of spodumene show strong
_pleochroism_, or the ability to show three different colors when viewed
in the direction of different axes. Some of the large Brazilian kunzite
crystals mined in the early 1960’s have an intense rose-violet color
when viewed along the long axis of the crystal but have pale blue-violet
and pale tan colors when viewed from the other two directions. When heat
treated, or exposed to strong light, this Brazilian kunzite loses its
tan and bluish colors but retains the intense rose-violet. Because of
spodumene’s pleochroism, the direction of cutting in the stones becomes
extremely important, as it must be done in a manner that will take
advantage of the violet color in kunzite and the green color in
hiddenite.
VARIETIES
Kunzite: Lavender violet to rose violet
Hiddenite: Deep green
GARNET
The name garnet is applied to a group of six closely related silicate
minerals that are alike in crystal structure but that differ mainly in
the substitution of certain metallic elements in their composition.
These minerals are:
_Pyrope_, magnesium aluminum garnet
_Almandine_, iron aluminum garnet
_Spessartine_, manganese aluminum garnet
_Uvarovite_, calcium chromium garnet
_Grossular_, calcium aluminum garnet
_Andradite_, calcium iron garnet
Most natural garnets have compositions intermediate between members of
the basic group of six. For example, there are garnets having
compositions anywhere between pyrope and almandine, depending on the
amount of difference in the magnesium or iron content. These same
garnets may even have varying amounts of manganese, and thus be
partially spessartine.
The six garnets in the basic group are found in considerable quantity in
many areas, but seldom are they of sufficiently high quality to be
considered gemstone material. Even when stones of gem quality are found,
their colors—particularly the reds—tend to be so intense that they seem
to be opaque.
[Illustration: Garnets occur in several colors, although most people
think of them as red. Shown here are a 54-carat spessartine from
Brazil (top right), a 6-carat rhodolite from North Carolina (at
left), a magnificent 10-carat green demantoid from Russia, a 9-carat
grossular from Ceylon (bottom), and a 26-carat spessartine from
Virginia. (Seven-eighths actual size.)]
Garnet has a hardness (about 7) suitable for gemstone material and a
fairly high refractive index (1.74 and above).
Ruby red pyrope is the most popular variety of garnet. It is found in
Bohemia, in Czechoslovakia, where it occurs as small, poorly shaped
crystals. Red pyrope also is found in Africa, where it is called Cape
ruby, and in Arizona, where it is sold as Arizona ruby. Another kind of
pyrope called _rhodolite_ is noted for its soft, rosy purple color.
Actually, rhodolite is one of the intermixed garnets with a composition
somewhere between pyrope and almandine. Most of the fine rhodolite gems
have come from North Carolina.
Almandine is popular in its deep red, transparent form, but since the
red is so dark and intense that it appears black, the stones usually are
cut as cabochons with the back hollowed out. This makes them thinner,
and thus lightens their color. Garnets cut in this manner are all known
as carbuncles. Brazil, India, Ceylon, Australia, and parts of the United
States are important sources of almandine.
Although spessartine has a rich orange color, it is not often used as a
gemstone because of the relative rarity of gem-quality cutting material.
This mineral gets its name from the town of Spessart, Germany, where it
was first found. Excellent spessartine with colors ranging from orange
to brown has been found at Amelia Court House, Va., and quality gems
have been cut from such material. Ceylon, Burma, Madagascar, and Brazil
also have furnished some gem spessartine.
The chromium garnet, uvarovite, generally is too poor in quality for
cutting. Uvarovite crystals, which are emerald green in color, occur in
only small sizes. They are found mostly in Russia, Finland, and
California.
Grossular varies in color. It occurs chiefly in some shade of red,
green, yellow, or brown, depending on the impurities present. When pure,
grossular is colorless. A kind of grossular called _hessonite_ has an
attractive cinnamon color, and it is found mainly in Ceylon. Because of
its color it can easily be confused with spessartine, which it closely
resembles.
Andradite, a very common garnet, usually is found in shades of red,
black, brown, yellow, or green. Some types of gem andradite have special
names for different colors: _topazolite_, yellow; _demantoid_, green;
and _melanite_, sparkling black. The very valuable demantoid is found in
Russia and Italy.
VARIETIES:
Grossular: Colorless, green, amber, brownish yellow, rose
Hessonite: Cinnamon colored
Pyrope: Deep red
Rhodolite: Rose red and purple
Almandine: Deep red
Spessartine: Brownish red to orange
Andradite: Yellow, greenish yellow, emerald green, brownish red,
brownish yellow, brown, black
Topazolite: Yellow to greenish
Demantoid: Grass green to emerald green
Melanite: Black
Uvarovite: Green
JADE
The name jade is applied to two unrelated minerals—_nephrite_ and
_jadeite_—that have somewhat similar characteristics.
Jadeite, the rarer of the two, is a sodium aluminum silicate that
belongs to a group of rock-forming minerals known as pyroxenes. Its
color varies from white to emerald green and many other colors. Jadeite
is highly prized, and when it occurs as emerald green it is considered
one of the most valuable gemstones. This kind of jade is found in many
places, but the most important occurrence is in Upper Burma. Nephrite, a
more common species, is a calcium magnesium iron silicate belonging to a
group of rock-forming minerals known as amphiboles. The color varies
from white to a dark spinach green and black. Among the places where
nephrite occurs are New Zealand, Turkestan, Siberia, Alaska, China,
Silesia, and certain parts of the western United States, notably in
Wyoming and California.
[Illustration: This emerald green jadeite carving, dating from the
Ch’ien-lung period (1736-1795), stands 6½ inches without the base.
It was given to the Smithsonian as part of the Maude Monell Vetlesen
collection.]
Jade is not particularly hard (6½), but it is very tough, and this
characteristic makes it an excellent material for carving. Even when
subjected to punishing usage, jade resists chipping and wear. It was
used for making tools and weapons by primitive peoples who lived in what
is now Mexico, Switzerland, France, Greece, Egypt, Asia Minor, and in
other places. The jade implements fashioned by these peoples have
survived well the ravages of time.
The Chinese and Japanese prize jade highly. In their countries,
tradition has assigned to jade medicinal and spiritual values, and has
associated with it the cardinal virtues of charity, modesty, courage,
justice, and wisdom. As a consequence, these peoples long ago developed
the carving of jade as a high art. Among the magnificent Chinese jade
carvings in the National Gem Collection are 130 pieces produced mostly
during the Ching Dynasty (1644-1912), when the art of jade carving was
at its peak. Many of these jades were carved in imitation of the revered
bronze ceremonial vessels of ancient times. This collection was
presented to the Smithsonian Institution in 1959 by Mr. Edmund C. Monell
in behalf of the estate of his mother, Mrs. Maude Monell Vetlesen of New
York.
[Illustration: This pale green jade vase of the Ch’ien-lung period
is 14½ inches high without the base. It is one of a matched pair
presented as part of the Maude Monell Vetlesen collection of carved
jade.]
CHARACTERISTICS OF SOME COMMON GEMS
Approximate average of
(1) hardness
(2) specific gravity (4) Dispersion
(3) refractive index (5) Durability
Species (1) (2) (3) (4) (5) Usual color range
Beryl 7¾ 2.70 1.58 Low High Green (emerald), blue-green
(aquamarine), pink (morganite),
colorless (goshenite)
Chrysoberyl 8½ 3.71 1.75 Low High Yellow, green, brown
Corundum 9 4.00 1.77 Low High Red (ruby), various (sapphire)
Diamond 10 3.52 2.42 High High Colorless
Garnet group 7½ 3.70- 1.74- Medium High Yellow, red, green, brown
4.16 1.89 to high
Jade 6½ 2.96 1.62 None High Green, white
(nephrite)
Jade 7 3.33 1.66 None High Green, white
(jadeite)
Opal 6 2.10 1.45 None Low Red, dark gray, orange, white,
with or without varicolored fire
Pearl 3½ 2.71 None None Low White
Peridot 6½ 3.34 1.68 Low Medium Yellow-green, brownish green
Quartz 7 2.65 1.55 Low High Purple (amethyst), yellow
(citrine), colorless (rock
crystal)
Spinel 8 3.60 1.72 Low High Shades of red, green, blue,
violet
Spodumene 7 3.18 1.66 Low Low Colorless, pink, yellow, green
Topaz 8 3.54 1.63 Low Medium Colorless, sherry, pink, blue
Tourmaline 7 3.06 1.63 Low High Wide range, except bright red
Zircon 7 4.02 1.81 High High Almost colorless, blue, brown,
green, yellow
GEMSTONES FOR THE COLLECTOR
A number of mineral species have produced cut gemstones that fulfill
every necessary requirement of beauty, durability, and rarity, but their
popularity and commercial success have been sharply limited because of
insufficient supply. In some cases of even adequate supply such
gemstones do not compete with other, more plentiful kinds that exhibit
the same characteristics. The scarcity of these minerals does not
diminish their standing as potential gem material—it merely points up
the effect of accidental natural distribution of these species.
[Illustration: A magnificent set of 16 matched sphenes from
Switzerland, gift of Nina Lea, almost encircles a 110-carat
sinhalite (a rare magnesium borate) and a 22-carat kornerupine, both
from Ceylon. The man’s gold ring indicates the sizes of these
unusual stones.]
Among the rarer minerals that produce good gemstones are cordierite,
benitoite, euclase, phenakite, beryllonite, willemite, wernerite,
danburite, datolite, axinite, brazilianite, andalusite, sillimanite,
kyanite, kornerupine, enstatite, diopside, epidote, sphene, sinhalite,
and orthoclase. Willemite, a rare zinc silicate found in only a few
localities, is typical of these rarer minerals. The famous zinc mines at
Franklin, N. J., produced a few large gemmy crystals of willemite, and
some fine gemstones were cut from some of these. Willemite’s borderline
hardness of 5 to 5½ and its extreme rarity effectively eliminate it from
the gem market, but the collector who is able to obtain a good stone of
this material is indeed fortunate.
[Illustration: Exotic gems that represent collectors’ items lie
beside a 3¼-inch-long box of Russian lapis lazuli. The stones are
(left row, from top) a 28-carat andalusite from Brazil, gift of Fred
C. Kennedy, a 10-carat cordierite from Ceylon, a 29-carat apatite
from Burma, and (right row) a 42-carat brazilianite from Brazil, a
13-carat euclase from Brazil, a 29-carat wernerite from Brazil, and
a 61-carat orthoclase from Madagascar.]
Some mineral species, although beautiful when cut, and prized by
collectors, are entirely too soft, are too easily cleaved, or have some
other physical weakness that renders them useless as commercial
gemstones. Sphalerite, apatite, fluorite, calcite, cerussite, zincite,
and hematite are included in this group. Sphalerite is typical; it
produces brilliant and colorful gemstones that hold their own among
other stones of great beauty. Unfortunately, this zinc sulfide, with a
hardness of 3½ to 4, is so soft and cleaves so readily that it is very
difficult to cut properly, and it cannot be used in jewelry.
7
SOME NOTABLE GEMS IN THE COLLECTION
The Smithsonian’s collection of gems continues to grow and improve
rapidly, and it changes character constantly as important new gemstones
are added and less important ones are retired. Approximately one-third
of the gems in the collection in 1965 are itemized in the following
list. Included are some of the largest gems of each kind, some of the
more interesting stones, and some small gems notable for the places from
which they came. Though listed by species and size, some of the larger
stones are not included, and neither are most cabochons, rough opal,
beads, carvings, and spheres. The descriptions listed include, in order,
weight in carats; color; popular name or other description, if any;
place of origin; and U. S. National Museum catalog number and name of
donor. Gems in the Lea and Roebling collections usually are indicated by
the letters “L” and “R.”
DIAMOND
127, colorless (_The Portuguese_), Brazil (3398)
44.5, blue (_The Hope_), India (3551, Winston)
18.3, yellow (_The Shephard_), South Africa (3406)
2.9, pink, Tanzania (3772, De Young)
CORUNDUM: Ruby
50, red-violet (a star), Ceylon (173, L)
34, red (a star), Ceylon (1922, L)
CORUNDUM: Sapphire
330, blue (_Star of Asia_), Burma (3688)
316, blue (_Star of Artaban_), Ceylon (2231, Ingram)
93, yellow, Burma (3549)
52, yellow, Burma (3419)
40, blue (a star), Ceylon (174, L)
35, yellow-brown, Ceylon (2147, L)
26, gray (a star), Ceylon (3902)
26, colorless, Ceylon (2016, L)
25, blue (4-starred), Ceylon (3923, Krandall)
22, yellow-orange, Ceylon (3875, L)
16, colorless, Ceylon (3581, L)
BERYL: Emerald
157, green, India (3601)
117, green, Colombia (4158, Erickson)
27, green, Colombia (3922)
17, green (3920, MacVeagh)
7, green, North Carolina (3075, L)
4.6, green (a cat’s-eye), Colombia (2256, R)
BERYL: Aquamarine
1000, green, Brazil (3889, Evyan)
264, blue, Russia (3606, Neal)
187, blue, Brazil (3683)
126, blue, Brazil (4159, Erickson)
71, pale blue, Ceylon (3172, L)
66, pale blue-green, Maine (2148, L)
15, blue-green, Idaho (2249, Montgomery)
14, blue, Connecticut (779)
10, blue, North Carolina (776, L)
BERYL: Morganite
236, pink, Brazil (3780, Ix)
122, pale pink, California (1988, R)
80, pale pink, Brazil (4190, R)
64, pink, Brazil (3721, R)
56, pink, Madagascar (2223, R)
51, pink, Brazil (3623)
BERYL: Beryl
2054, green-gold, Brazil (3725, R)
1363, green, Brazil (3916)
914, green, Brazil (3919)
578, green, Brazil (3227, R)
133, yellow, Madagascar (1977, L)
114, yellow-green, Brazil (2245, R)
98, pale green, Brazil (3949, Cutter)
62, colorless (goshenite), Brazil (3366)
46, gold, Madagascar (2121, L)
44, gold (a cat’s-eye), Madagascar (3248)
40, pale green, Connecticut (1037, L)
40, yellow-green, North Carolina (2260, Roebling)
20, brown (a star), Brazil (3355, L)
TOPAZ
7725, yellow, Brazil (3976)
3273, blue, Brazil (3633)
1469, yellow-green, Brazil (3891)
685, pale blue, Brazil (3003)
398, pale blue, Russia (3400, R)
235, colorless, Colorado (3309, L)
187, colorless, Brazil (3612, Cutter)
171, champagne, Madagascar (3890)
155, blue, Russia (262, L)
146, pale blue, Texas (3625, L)
129, sherry, Brazil (3550)
94, orange, Brazil (3401, R)
54, blue, Brazil (2219, L)
51, colorless, Japan (268)
44, blue, Maine (2047, L)
41, orange, Brazil (2174, L)
34, gold, Brazil (2046, L)
34, deep pink, Brazil (2232, L)
24, pale blue, New Hampshire (3307, L)
18, rose pink, Brazil (3402, R)
17, blue, California (3679, Ware)
15, sherry, Colorado (318, L)
TOURMALINE: Rubellite
111, pink, Manchuria (3173, R)
62, pink, Brazil (3411, R)
51, magenta, Brazil (4160, Erickson)
35, pink, Brazil (2254, R)
34, pink, Brazil (3148, R)
30, pink, Madagascar (3409, R)
18, pink (a cat’s-eye), California (3786, Lea)
18, pink, Maine (1109, L)
15, pink, California (3412, R)
TOURMALINE: Tourmaline
173, champagne, Mozambique (3590, R)
125, champagne, Mozambique (3576, R)
123, green, Mozambique (3575, R)
110, green, Brazil (4197)
104, rose, Mozambique (3256, L)
76, dark green (a cat’s-eye), Brazil (3599, L)
60, blue-green, Brazil (3410, R)
58, green, Maine, (1108, L)
53, green (a cat’s-eye), Brazil (3119, L)
48, red and green, California (3363)
42, yellow, Brazil (2251, R)
42, brown, Ceylon (3245, L)
40, red-brown, Brazil (2097, R)
40, green, Madagascar (4081, R)
34, red-brown, Brazil (2253, L)
31, rose-brown, Brazil (3416, R)
26, blue (indicolite), Brazil (3298, R)
20, blue-green, Madagascar (2032, L)
18, yellow-green, Elba (3368, R)
18, green, South Africa (2095, L)
15, yellow, Brazil (3415, R)
SPINEL
46, pale purple, Ceylon (2180, L)
36, indigo, Burma (3685)
34, red, Burma (3354, L)
30, pink-violet, Ceylon (2165, L)
30, violet, Burma (3344, L)
26, blue-gray, Burma (3593, L)
22, blue-violet, Ceylon (2247, R)
22, rose-brown, Ceylon (2166, L)
ZIRCON
118, brown, Ceylon (2236, R)
106, brown, Thailand (3568)
103, blue, Indochina (2222, R)
98, yellow-brown, Ceylon (2237, R)
76, red-brown, Burma (3068, L)
64, brown, Indochina (3397, R)
48, colorless, Ceylon (3554, L)
29, blue, Indochina (3394, R)
23, green, Ceylon (2233, R)
21, tan, Australia (1887, L)
SPODUMENE: Kunzite
830, deep violet, Brazil (3940)
336, deep violet, Brazil (3942, Nelson)
297, deep violet, Brazil (3941, Nelson)
177, violet, California (3797, American Gem Society)
25, pale violet, Madagascar (1979, L)
SPODUMENE: Spodumene
327, yellow, Brazil (3396, R)
256, yellow, Brazil (3429, R)
71, yellow, Madagascar (3698, L)
69, yellow-green, Brazil (3885, R)
PERIDOT
310, olive green, Egypt (3398, R)
287, olive green, Burma (3705)
46, olive green, Egypt (1978, L)
23, olive green, Arizona (3620, L)
GARNET: Almandine
175, red (a star), Idaho (3670)
67, red-brown (a star), Idaho (3560, L)
41, red-brown, Madagascar (2137, L)
26, red-brown, Idaho (3423, L)
GARNET: Demantoid
10.4 green, Russia (2175)
GARNET: Grossular
64, orange-brown, Ceylon (493, L)
GARNET: Rhodolite
25, rose-violet, Tanzania (4080, L)
6.4, violet, North Carolina (460, L)
GARNET: Spessartine
109, red, Brazil (4203)
40, orange, Virginia (147, L)
26, orange, Virginia (3597, L)
QUARTZ: Amethyst
1362, purple, Brazil (3879)
183, purple, Brazil (1272, L)
62, purple, Brazil (3162, Capps)
61, purple, Brazil (3914, Cutter)
56, purple, Brazil (3165, Capps)
54, purple, Pennsylvania (1299, L)
45, pale purple, North Carolina (1298, Lea)
36, purple, Pennsylvania (1283, L)
33, pale purple, North Carolina (1288, Lea)
27, purple, Arizona (3291, R)
23, purple, Maine (1271, L)
19, purple, Virginia (1301, L)
QUARTZ: Citrine
1180, golden brown, Brazil (1870, L)
783, light golden brown, Brazil (3640)
278, golden brown, Brazil (3732, Cutter)
265, light golden brown, Brazil (2041, Roebling)
218, golden brown, Brazil (4199, Cutter)
169, golden brown, Australia (1373, L)
143, yellow, Colorado (456, L)
120, golden brown, Brazil (2116, L)
115, golden brown, Brazil (3932)
91, yellow, Brazil (3615, Cutter)
55, light golden brown, Maine (2178, L)
48, yellow, Brazil (3915, Cutter)
43, yellow, Brazil (3719, Cutter)
QUARTZ: Rock Crystal
7000, colorless, Brazil (3957, R)
625, colorless (a star), New Hampshire (3125, Burroughs)
350, colorless, North Carolina (1398, L)
QUARTZ: Rose Quartz
375, pink, Brazil (3592, L)
84, pink, Brazil (3421)
49, pink, Brazil (3420, R)
QUARTZ: Smoky Quartz
4500, pale smoky, California (3738, L)
1695 smoky, Brazil (3697, L)
785, pale smoky, Colorado (1335, L)
284, pale smoky, North Carolina (1340, Lea)
163, pale smoky, Colorado (1336, L)
145, smoky, Scotland (3079, R)
CHRYSOBERYL: Alexandrite
66, green to red, Ceylon (2042, L)
17, green to red, Ceylon (3407, R)
11, green to red, Ceylon (2200, Walcott)
CHRYSOBERYL: Chrysoberyl
172, gray-green (a cat’s-eye), Ceylon (3924)
121, green (_The Maharani_, a cat’s-eye), Ceylon (3642)
46, green-yellow, Brazil (1923, L)
32, brown, Ceylon (2151, L)
OPAL
155, white with fire, Australia (3285, Roebling)
83, white with fire, Australia (3300, R)
58, black with fire, Australia (3960, R)
56, colorless with fire, Mexico (2240, R)
54, black with fire, Australia (3962)
44, black with fire, Australia (3284, R)
39, pale yellow-orange with fire, Brazil (3637)
38, black with fire, Australia (3961)
30, black with fire, Australia (3405, R)
24, black with fire, Australia (1897, L)
22, orange with fire, Mexico (2106, L)
22, orange with fire, Mexico (2028, L)
21, yellow with fire, Mexico (2111, L)
15, orange with fire, Mexico (2096, L)
11, orange with fire, Mexico (3886, Lewis)
OTHER, LESS-KNOWN SPECIES
Albite: 43, white (a cat’s-eye), Burma (3311, L)
Amblygonite: 63, yellow, Brazil (4079, Lea)
20, yellow, Burma (3562, R)
Andalusite: 28, brown, Brazil (3619, Kennedy)
14, green-brown, Brazil (3364, L)
Apatite: 29, yellow-green, Burma (3247, Lea)
29, yellow, Mexico (3594, L)
15, colorless, Burma (3720, R)
9, yellow-green, Canada (3122, R)
8.8, pale blue, Ceylon (3639)
5.4, green, Madagascar (3676, Durand)
Axinite: 9.4, brown, Mexico (3787, R)
9, brown, Mexico (3773, L)
Barite: 61, colorless, England (3349)
Benitoite: 7.6, blue, California (3387, R)
Beryllonite: 5, colorless, Maine (423)
Brazilianite: 42, yellow, Brazil (3083, L)
Calcite: 46, gold-brown, Mexico (3305)
Cassiterite: 10, yellow-brown, Bolivia (3250)
Cobaltocalcite: 3.3, 3.9, pink, Spain (3724, L)
Cordierite: 16, blue, Ceylon (3882)
10, indigo, Ceylon (3580, L)
9.4, blue, Ceylon (3881)
Danburite: 18, yellow, Burma (3345, L)
7.9, colorless, Japan (3801, L)
Datolite: 5.4, colorless, Massachusetts (3876, Boucot)
5, colorless, Massachusetts (3283, Sinkankas)
Diopside: 133, black (a star), India (3977)
24, black (a cat’s-eye), India (3956, Lea)
14, black (a cat’s-eye), India (3880)
11, green, Madagascar (2264, R)
6.8, yellow, Italy (3634)
4.6, yellow, Burma (3346, L)
2.2, pale green, New York (572, L)
1.6, green (chrome diopside), Finland (3693)
Enstatite: 11, brown, Ceylon (3638)
8.1, brown, Ceylon (2294, R)
Epidote: 3.9, brown, Austria (579)
Euclase: 13, green, Brazil (3214, R)
9, yellow, Brazil (3215, R)
8.9, yellow, Brazil (2181, L)
3.7, blue-green, Brazil (3388, R)
Fluorite: 354, pale yellow, Illinois (3877)
125, green, New Hampshire (3294)
117, green, Africa (2153)
63, yellow, Illinois (3595, L)
33, colorless, Illinois (3626)
8.5, pink, Switzerland (3730, R)
Friedelite: 12, red-brown, New Jersey (3013, D’Ascenzo)
Gadolinite: 8.6, black, Texas (587, L)
Idocrase: 3.5, brown, Italy (4179, R)
Kyanite: 11, blue, Brazil (3557, L)
9.1, green, Brazil (3558, L)
3.7, blue, North Carolina (364, Bowman)
Kornerupine: 22, brown, Ceylon (3706, Lea)
11, brown, Madagascar (3567, L)
7.6, green, Madagascar (3782)
Labradorite: 11, pale yellow, Utah (3121)
Microlite: 3.7, brown, Virginia (3588, Lea)
Oligoclase: 6, colorless, North Carolina (404, L)
Orthoclase: 250, yellow, Madagascar (3878)
105, pale green (a cat’s-eye), Ceylon (3883)
61, yellow, Madagascar (1838, L)
26, gray (a cat’s-eye), Ceylon (3579, Lea)
23, white (a star), Ceylon (3578, L)
Petalite: 11, colorless, South-West Africa (3096)
Phenakite: 22, colorless, Russia (3739)
10, colorless, Brazil (2263, R)
Phosphophyllite: 5, green, Bolivia (3950, Roebling)
Pollucite: 9, colorless, Maine (2056, L)
7, colorless, Connecticut (3802, R)
Proustite: 9.9, red, Germany (4082, L)
Rhodizite: 0.5, colorless, Madagascar (3219, Canfield)
Rhodochrosite: 9.5, pink, South Africa (4189, L)
Samarskite: 6.6, black, North Carolina (588, L)
Scheelite: 37, colorless, California (3701, L)
12, gold, Mexico (3803, R)
Scorodite: 2.6, purple, South-West Africa (3793)
Sillimanite: 5.9, black (a cat’s-eye), South Carolina (3600, L)
Sinhalite: 110, brown, Ceylon (3587)
44, brown, Ceylon (3548, L)
Sphalerite: 73, yellow-brown, Utah (3556)
69, yellow-brown, Utah (3362)
60, yellow-green, New Jersey (3874, Roebling)
48, yellow, Mexico (2167, L)
46, yellow, Spain (3707, L)
Sphene: 0.8-9.3, sixteen stones, gold, Switzerland (2043, Nina Lea)
8.5, brown, New York (550)
5.6, yellow-brown, Mexico (3290)
5.2, yellow-brown, Mexico (3292)
Staurolite: 3, dark red-brown, Brazil (3795)
Tektite: 23, brown, Czechoslovakia (681, L)
Wernerite: 288, colorless, Burma (3783)
30, colorless (a cat’s-eye), Burma (3301, L)
29, pale yellow, Brazil (2098, L)
17, pink (a cat’s-eye), Ceylon (3238, Roebling)
12, pink, Burma (3674, L)
Willemite: 12, orange-yellow, New Jersey (1898, L)
11, orange-yellow, New Jersey (4187, Lea)
Zincite: 20, red, New Jersey (3386, R)
[Illustration: Seal of the Smithsonian Institution]
Transcriber’s Notes
—Silently corrected a few typos.
—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
—In the text versions only, text in italics is delimited by
_underscores_.
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