Gems
From Encyclopædia
Gemstones are unusually bright, colorful, or transparent minerals found in the rocks of the Earth. Pearl,
coral,
amber, ivory,
shell, jet, and similar materials are also often considered gemstones, although they are the end products from processes related to living
plants or animals.Gemologists use several different tests to identify gemstones. A specially designed gem refractometer determines the refractive
index--a measure of
light-bending ability equal to the
sine of the angle of incidence divided by the
sine of the angle of refraction. Each species of gem has its own particular refractive
index. The detection under a microscope of foreign inclusions and the determination of hardness by tiny test scratches made with special hardness pencils are usually sufficient additional tests.
density determination, or, more rarely, sophisticated methods of
X-ray diffraction and chemical analysis, are sometimes also required.Only about 100 mineral species (out of an estimated 3,000 known) have been found reasonably suitable for use as gems. Approximately 13 are normally found circulating widely in the gem trade, and perhaps 40 more are sometimes available to the gem collector. The remainder are mostly decorative or ornamental stones of high beauty but relatively low market value.The cutting of gems is both a craft and an art (see GEM CUTTING). Although the techniques of carving and sculpting mineral materials date from the 3d millennium BC, optical cutting of gems began only in the late 1600s with the devising of the brilliant cut by Vincenzio Vincenti Peruzzi of
Venice. Although irregularly shaped, smoothed, or tumbled gemstones have returned to fashion in our time, the finest pieces of gem rough are converted primarily into patterned, faceted stones. Various combinations of flat faces or facets have been devised, all designed to bring out of the gem the greatest brilliance or
color display possible. Emeralds and other attractively colored gems have different kinds of cuts, usually with large flat table facets, to display the interior
color at its best.WHAT MAKES A GEM DESIRABLEIn various cultures throughout history, certain preferences have developed for one kind of gemstone over another. These preferences, reflected in commercial price levels for gems, are based on several factors: beauty, durability, rarity, and the current fashion. Diamond or the finest
jade come as close as any gemstones to meeting all these requirements. Some gemstones meet only a minimum of these requirements; for example, turquoise is beautiful and fashionable but neither particularly rare nor durable.The beauty of a gemstone, usually revealed to its fullest only after cutting and polishing, may depend on the natural
color, transparency, or some other more unusual optical effect. Most important gemstones are allochromatic; that is, the
color of an individual stone depends on very slight differences in composition. Those whose
color does not depend on these compositional impurities are idiochromatic. Some gemstones, because of their chemical and physical nature, are always opaque. The beauty of turquoise can be seen only by
light reflected from polished surfaces. Other gems are quite transparent, transmitting
light easily. The importance and beauty of transparent gemstones may be greatly diminished, however, if they contain internal flaws or bits of foreign material; blemishes are clearly visible. Totally clean, high-quality gemstones are exceedingly rare, because minerals are usually formed in nature under
stress or from relatively impure materials. Inclusions of
rutile,
spinel,
carbon, actinolite, and numerous other minerals are common.REFRACTION, DISPERSION, DIFFRACTION, and REFLECTION are other important optical effects that produce beauty in gemstones. Different
colors, or wavelengths of
light, are bent, or refracted, by different amounts as they travel from the air into a cut gem. When white
light, a mixture of all wavelengths, enters certain gems, its
colors are dispersed or spread by differential refraction and are then reflected back out of the gem as separate
color flashes. This phenomenon is quite familiar in diamond, which has excellent dispersion.
colors are also separated by diffraction, an optical process that depends on the orderly internal array of gemstone
atoms or groups of
atoms; among gems it is best seen in
opal.Sample reflection of
light from internal flaws and inclusions in gems can also cause some remarkable and beautiful effects.
star sapphires and rubies, for example, contain thousands of needlelike foreign particles that line up uniformly in the three major directions of atomic structure for these species.
light reflecting from these three sets of needles produces asterism, a bright six-rayed, starlike reflection. If the needlelike inclusions line up in only one direction, as t?ey sometimes do in the mineral species
chrysoberyl, the reflection is in only a single ray, giving a CAT's-EYE effect, or chatoyancy.The durability of a gemstone depends on its toughness (its freedom from a tendency to break, cleave, or otherwise separate) and its hardness (its resistance to scratching and general abrasion).
jade is tough but only moderately hard; diamond, on the other
hand, is hard but only moderately tough. Gemstones usually exceed 6 on Mohs' scale of hardness (see HARDNESS, MINERAL) and often exceed 7 or more. No comparable established standard scale for toughness exists.The rarity of gemstones depends on two factors. Some mineral materials, such as apexandrite, just do not exist in nature in large quantities. Others, such as
opal, are common, but only a small
number have enough quality and beauty to be considered for gem purposes.Fashion is one of the least predictable factors determining gemstone desirability and value. Popularity may depend on the availability and price of gems, or perhaps on a volatile fadlike preference. Diamond, ruby,
sapphire, emerald, and pearl are traditionally considered as precious stones, although that term today is almost meaningless.
opal, amethyst,
amber, periodot, turquoise, jet, zircon, citrine, and others seem to drift in and out of fashion.POPULAR GEMSThe most popular and therefore the most important commercial gems include diamond, ruby,
sapphire, emerald and pearl. DIAMOND--at 10 on Mohs' scale, the hardest of all known substances--is composed of
carbon and comes mostly from South
Africa and
Russia. Though most highly prized when colorless, it can be yellow, blue,
pink, or other
colors. RUBY and
sapphire are distinct varieties of the mineral species CORUNDUM, an aluminum oxide. Ruby is colored red by traces of
chromium; the various
colors of
sapphire, however, are produced by traces of
iron and titanium in the corundum. The best gem corundum is found in Sri Lanka and in the Orient at places such as Mogok in Upper Burma, near
Bangkok in
Thailand, and Kashmir in India. EMERALD, the rich blue green variety of the mineral species
beryl (a
beryllium-aluminum silicate), gets its
color from traces of
chromium and a little
iron. The best emeralds are from the Muzo and Chivor
mines in
Colombia.
fine pearls are the products of only a certain genus of
oysters, Pinctada, which deposit layers of pearl over irritants that have gotten inside their
shells (see
PEARLS AND PEARLING).A typical
jewelry store may carry several other kinds of gems: blue to bluish green aquamarine,
pink to peach morganite, and other varieties (including those colored yellow green to green) of
beryl; the complex borosilicate
tourmaline, which may be
pink to red (rubellite), blue (indicolite), bright green or several other
colors; the aluminosilicate
topaz, which may be sherry or muscatel wine colored,
pink, green, blue, or brown;
quartz (
silicon dioxide), whose gem varieties include AMETHYST (purple), chrysoprase (green), citrine (yellow to brown), rock
crystal (colorless), and rose
quartz; and
opal, a stone often with brilliant
color flashes that is usually dark and white (
Australia) or transparent (anywhere from colorless to orange-to-red and found in
Mexico).Less common gems include the
beryllium aluminate
chrysoberyl, especially its cat's-eye variety and the fascinating alexandrite variety, which appears green in daylight and red in artificial incandescent
light; GARNET, such as the red Bohemian pyropes and Indian almandines;
peridot, a yellow green variety of the mineral species OLIVINE, a magnesium-
iron silicate; the magnesium-aluminate
spinel, in its ruby red and mauve varieties; TANZANITE, a
sapphire blue variety of the mineral species
zoisite, a calcium-aluminum hydroxy silicate; and spodumene, in its green to yellow green varieties or the beautiful
lavender lilac kunzite variety.Numerous other good gem species are sometimes available for collectors, including
cordierite,
sphene,
andalusite, benitoite and euclase. Ornamental gemstones, beautiful but not suited for f?shioning into transparent cut gems, include
coral, nephrite and jadeite
jade, JET,
lapis lazuli,
malachite,
rhodonite, SERPENTINE, TURQUOISE, and various varieties of
quartz including AGATE,
jasper, and carnelian.COUNTERFEIT GEMSBoth now and in past centuries far more substitute and artificial gemstones have been in circulation than natural, properly identified stones. Gemstone counterfeiting is not normally intended for fraud, and fraud does not usually occur because of the materials or their manufacturers; rather it comes about from the motives of the buyer or seller. Gemstone counterfeiting caters to a market of thousands of consumers who feel they cannot afford the price of an attractive natural gem. Also, many buyers who can afford them prefer to invest in other sorts of luxury goods.
