GEM

(mineralogy) A natural or artificially produced mineral or other material that has sufficient beauty and durability for use as a personal adornment.

A mineral or other material that has sufficient beauty for use as personal adornment and has the durability to make this feasible. With the exception of a few materials of organic origin, such as pearl, amber, coral, and jet, and inorganic substances of variable composition, such as natural glass, gems are lovely varieties of minerals.

Natural gems

Each distinct mineral is called a species by the gemologist. Two stones that have the same essential composition and crystal structure but that differ in color are considered varieties of the same species. Thus ruby and sapphire are distinct varieties of the mineral species corundum, and emerald and aquamarine are varieties of beryl. See also Mineral; Mineralogy.

Most gemstones are crystalline (that is, they have a definite atomic structure) and have characteristic properties, most of which are related directly to either beauty or durability. Each mineral has a characteristic hardness (resistance to being scratched) and toughness (resistance to cleavage and fracture). With few exceptions, the most important gemstones are those at the top of the Mohs hardness scale; for example, diamond is 10, ruby and sapphire are 9, chrysoberyl is 8½, and topaz, beryl (emerald and aquamarine), and spinel are 8.

Optical properties are particularly important to the beauty of the various gem materials. The important optical properties include color; dispersion (or “fire”); refractive index (relating the breaking up of white light into colors—a rough measure of brilliancy); and pleochroism (the property of some doubly refractive materials of absorbing light unequally in the different directions of transmission, resulting in color,differences). Gemstones usually are cherished for their color, brilliancy, fire, or one of the several optical phenomena, such as asterism (the star effect caused by certain reflections of light); chatoyancy, or a cat's-eye effect; play of color, such as displayed by an opal; and adularescence (the billowy light effect seen in adularia or moonstone varieties of orthoclase feldspar).

Gemstones are commonly designated as precious or semiprecious. This is a somewhat meaningless practice, however, and often misleading, since many of the so-called precious gem varieties are inexpensive and many of the more attractive varieties of the semiprecious stones are exceedingly expensive and valuable. For example, a piece of fine-quality jadeite may be valued at approximately 100 times the price per carat of a low-quality star ruby. Fine black opals, chrysoberyl cat'-eyes, and alexandrites are often much more expensive than many sapphires of certain colors. See also Precious stones.

More than 100 natural materials have been fashioned at one time or another for ornamental purposes. Of these, however, only a relatively small number are likely to be encountered in jewelry articles.

The table lists the important gem minerals and the properties most useful in identification. For further information on the individual species or groups See also Amber; Amethyst; Azurite; Beryl; Cameo; Chrysoberyl; Corundum; Diamond; Emerald; Feldspar; Garnet; Intaglio (gemology); Jade; Jet (gemology); Labradorite; Lazurite; Malachite; Olivine; Onyx; Opal; Orthoclase; Pearl; Quartz; Ruby; Sapphire; Spinel; Spodumene; Topaz; Tourmaline; Turquoise; Zircon.

Manufactured gems

A mineral or other material that has sufficient beauty and durability for use as a personal adornment can be manufactured. The term “manufactured,” as used here, does not include such processes as shaping, faceting, and polishing, but only the processes that affect the material from which the finished gem is produced. These processes are (1) those that change the mineral in some fundamental characteristic, such as color, called a treated gem; (2) those by which a material is made that is identical with the naturally occurring mineral, called a synthetic gem; and (3) those that produce a simulated material with the appearance but not both the composition and structure of the natural gem, called an imitation gem.

Treated gems

There are four basic methods of treatment: (1) dyeing and staining, (2) plastic or other impregnation, (3) heat treatment, and (4) radiation. When the process to which a gem material is subjected changes its structure or adds something, such as a dye or a plastic binder, an effect on value takes place. When such changes are made, the nature of the alteration must be disclosed. An example of the second category of treatment, wherein there is no obvious effect on value, is gentle heating of amethyst to even its color, or stronger heating to change it to yellow or brown citrine. The change is permanent and nothing but temporary heat has been added. Many colored stones, including most green tourmaline, aquamarine, and colorless and flame-colored zircon and all pink topaz and blue zircon have been heated to improve their color.

Synthetic gems

The U.S. Federal Trade Commission has restricted the term synthetic gems to manufactured materials that have the same chemical, physical, and optical properties as their naturally occurring counterparts. Many gem materials, including diamond, have been synthesized, but in such small crystals or poor quality that they are unsatisfactory as gemstones. Some attempts to make gemstones have resulted in producing substances hitherto not known, many of which are of great importance industrially. Others have resulted in significant improvements in existing processes.

Imitation gems

Since prehistoric times glass has been the most widely used gem imitation. Since World War II colored plastics have replaced glass to a great extent in the least expensive costume jewelry.

Identification

Materials made by a flame-fusion process almost always contain gas bubbles, which are usually spherical or nearly so. Those with medium to dark tones of color often show color banding, or striae, with a curvature corresponding to that of the top of the boule. Natural gem materials are characterized by angular inclusions and straight color bands, if any are present.

Flux-fusion synthetic emeralds have distinctly lower refractive indices and specific gravities than natural emeralds and are characterized by wisplike or veillike flux inclusions. They show a red fluorescence under ultraviolet light, whereas most natural emeralds are inert.

Hydrothermally made synthetic emeralds have properties similar to many natural emeralds, but their inclusions differ and they are characterized by a very strong red fluorescence under ultraviolet.

The cheaper forms of glass are cast in molds and, under a hand lens, show rounded edges at the intersections of facets; the facets are often concave. The better grades, known as cut glass, have been cut and polished after first being molded approximately into the desired form. Cut glass has facets that intersect in sharp edges. Both types may contain gas bubbles or have a roiled appearance in the heart or have both, in contrast to most of the colored stones they imitate.

GEM may refer to:

• G.E.M., a Chinese pop singer from Hong Kong
• Gas Electron Multiplier
• Gender Empowerment Measure
• Generable (sic) Enigmatic Matrix from the anime and manga My-Otome (see List of My-Otome terminology)
• Generalized Empirical Method — from Bernard Lonergan, also known as critical realism
• Generalized Expertise Measure
• Generic Equipment Model - Generic Model for Communications and Control of Manufacturing Equipment
• Generalmusic — an Italian manufacturer of organs, synths and keyboards, formerly 'Galanti Electro Music'
• Genetically engineered microorganism
• German equatorial mount
• Global Earthquake Model
• Global Electric Motorcars - Manufacturer of electric cars and subsidiary of Chrysler
• Global Environmental Multiscale Model
• Global Entrepreneurship Monitor — the most comprehensive study of global entrepreneursip trends, conducted annually by the GEM Consortium, with 39 member countries
• Globally Executable MHP, a digital television specification
• Glycosphingolipid-enriched microdomains, in Biology. Also known as Lipid raft
• Gordon/Evernham Motorsports
• GPON encapsulation method — a method of encapsulation for GPON (Gigabit Passive Optical Network)
• Grand Egyptian Museum
• Graphical Environment Manager — a windowing system created by Digital Research, Inc. (DRI)
• Graphics Environment for Multimedia — a Pure data package for 3D graphics
• Graphics Execution Manager — a Kernel based graphics memory manager replacement for Translation Table Maps
• Graphite-Epoxy Motor — a solid rocket motor used for supplemental thrust on several space launch vehicles
• Gravitoelectromagnetism — deals with the theory of field effects due to moving masses
• Grenoble ecole de management — the Grenoble Graduate School of Management
• Ground effect machines also known as Hovercraft
• Growth Enterprise Market of Hong Kong Stock Exchange
• Gyrokinetic ElectroMagnetic A global electromagnetic gyrokinetic simulation code for studying tokamak turbulence

See also

• Gem (disambiguation)

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