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.