gem, ornamental mineral or organic substance

gem, commonly, a mineral or organic substance, cut and polished and used as an ornament. Gems also are used as seals (items of assurance) and as talismans (good-luck charms). For birthstones, see month month, in chronology, the conventional period of a lunation, i.e., passage of the moon through all its phases. It is usually computed at approximately 29 or 30 days.
..... Click the link for more information. .

Properties of Gems

The qualities sought in gems are beauty, rarity, and durability. The beauty of a gem depends primarily on its optical properties, which impart its luster, fire, and color; the durability depends on hardness and resistance to cleavage or fracture. The physical properties by which gems are distinguished from each other are form of the crystal, index of refraction of light, hardness, presence or absence of cleavage, type of fracture (conchoidal, even, or uneven) in stones without cleavage, specific gravity, color, streak (color of the powder as determined by rubbing it over white, unglazed porcelain), luster (appearance of the surface in reflected light—adamantine, vitreous, resinous, greasy, silky, or pearly), and transparency. Minor properties that serve to identify some stones are chatoyancy (changeable luster or color under undulating light), opalescence, asterism (starlike sparkling), play of color, fluorescence, phosphorescence, iridescence, and electrical properties. The unit of weight used for gemstones is the metric carat; one carat equals 200 mg.

Gem Cutting

Gems are generally cut to bring out their natural color and brilliancy and to remove flaws. In the cabochon cut, the upper surface of the stone is smoothed and rounded into a simple curve of any degree of convexity; the lower surface may be concave, convex, or flat. All the remaining cuts have flat facets. In the table cut, the facets of the natural octahedron of the diamond are ground to smoothness and polished; one facet, the table, is ground much larger than any other and made the top of the gem, while the opposite facet, the culet, is left quite small. The rose cut consists of a flat base and (usually) 24 triangular facets—resembling a cabochon with facets. The brilliant cut is scientifically designed to bring out the maximum brilliancy of the stone. The crown of a brilliant consists of a table and 32 smaller facets, of which 8 are quadrilaterals and 24 are triangles; the base, of a culet and 24 larger facets, of which 8 are quadrilaterals and 16 are triangles. The base and crown are separated by a girdle. The brilliant cut has certain proportions—in general, the depth of the crown is one third the depth of the stone and the width of the table one half the width of the stone. The trap, step, or emerald cut consists of a table and quadriangular facets above and below the girdle with parallel horizontal edges. Diamond cutting and the cutting of other precious stones are distinct trades.

Diamond Cutting

In diamond cutting the stone is first cleaved or sawed to remove excrescences (outcroppings) or to break it into smaller stones. Cleaving is accomplished by making a groove in the surface in the direction of the grain, inserting a steel knife, and striking the back of the knife a sharp blow. The next process was formerly bruting, i.e., roughly shaping two stones by rubbing them against one another. In modern practice the stones are sawed with a revolving wheel coated on its rim with diamond powder, then shaped by inserting a holder, or dop, containing one diamond into a turning lathe that revolves it against a stationary diamond. The cutting of the facets and the polishing are done by a revolving iron wheel charged with diamond dust. After the facets are cut, the diamonds are cleaned and are ready for sale.

Other Gemstones

The cutter of gemstones other than diamonds is known as a lapidary. Precious and semiprecious stones other than diamonds are cleaved or slit by a revolving diamond-dusted wheel, faceted by being pressed against a lap (a smoothing and polishing tool) charged with diamond dust or a carborundum wheel, and polished with a softer abrasive. Most (and in the case of some gems all) of the work of faceting is done with only the eye of the lapidary as guide.

Types of Gemstones

Precious and Semiprecious Gemstones

The precious stones are diamond diamond, mineral, one of two crystalline forms of the element carbon (see allotropy ), the hardest natural substance known, used as a gem and in industry.

Properties

..... Click the link for more information. ; some forms of corundum corundum (kərŭn`dəm), mineral, aluminum oxide, Al₂O₃.
..... Click the link for more information. (ruby ruby, precious stone, the transparent red variety of corundum , found chiefly in Myanmar, Thailand, and Sri Lanka and classified among the most valuable of gems. The Myanmarese stones are blood red, the most valued tint being the "pigeon's blood.
..... Click the link for more information. , sapphire sapphire, precious stone. A transparent blue corundum , it is classified among the most valuable of gems. Sapphires are found chiefly in Thailand, India, Sri Lanka, and Myanmar and also in Australia and in the United States (in Montana).
..... Click the link for more information. , Oriental emerald, Oriental topaz, and Oriental amethyst); and emerald emerald, the green variety of beryl , of which aquamarine is the blue variety. Chemically, it is a beryllium-aluminum silicate whose color is due to small quantities of chromium compounds.
..... Click the link for more information. . The chief semiprecious stones are aquamarine aquamarine (ăk'wəmərēn`, äk'–) [Lat.,=seawater], transparent beryl with a blue or bluish-green color.
..... Click the link for more information. , amethyst amethyst (ăm`əthĭst) [Gr.,=non-drunkenness], variety of quartz , violet to purple in color, used as a gem.
..... Click the link for more information. , topaz topaz (tō`păz), aluminum silicate mineral with either hydroxyl radicals or fluorine, Al₂SiO₄(F,OH)₂
..... Click the link for more information. , garnet garnet, name applied to a group of isomorphic minerals crystallizing in the cubic system. They are used chiefly as gems and as abrasives (as in garnet paper).
..... Click the link for more information. , tourmaline tourmaline (t
..... Click the link for more information. , spinel spinel, magnesium aluminum oxide, MgAl₂O₄, a mineral crystallizing in the isometric system, usually as octahedrons. It occurs as an accessory mineral in basic igneous rocks, in aluminum-rich metamorphic rocks, and in contact-metamorphosed
..... Click the link for more information. , peridot (see olivine olivine (ŏlĭv`ēn), an iron-magnesium silicate mineral, (Mg,Fe)₂SiO₄
..... Click the link for more information. ), zircon (see zirconium zirconium (zərkō`nēəm), metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.22; m.p. about 1,852°C;; b.p.
..... Click the link for more information. ), chrysoberyl chrysoberyl (krĭs`əbĕr'ĭl) [Gr.,=golden beryl], a beryllium aluminate used as a gem.
..... Click the link for more information. , quartz quartz, one of the commonest of all rock-forming minerals and one of the most important constituents of the earth's crust. Chemically, it is silicon dioxide, SiO₂.
..... Click the link for more information. , opal opal (ō`pəl), a mineral consisting of poorly crystalline to amorphous silica, SiO₂·nH₂
..... Click the link for more information. , turquoise turquoise, hydrous phosphate of aluminum and copper, Al₂(OH)₃PO₄·H₂O+Cu, used as a gem. It occurs rarely in crystal form, but is usually cryptocrystalline.
..... Click the link for more information. , moonstone moonstone, an orthoclase feldspar , found in Sri Lanka, Myanmar, and Madagascar (and formerly in the St. Gotthard district of Switzerland). In spite of its pronounced cleavage, it is widely used as a gem .
..... Click the link for more information. , and jade jade, common name for either of two minerals used as gems. The rarer variety of jade is jadeite, a sodium aluminum silicate, NaAl(SiO₃)₂, usually white or green in color; the green variety is the more valuable.
..... Click the link for more information. . The organic gems are pearl pearl, hard, rounded secretion formed inside the shell of certain mollusks, used as a gem. It is secreted by the epithelial cells of the mantle, a curtain of tissue between the shell and body mass, and is deposited in successive layers around an irritating
..... Click the link for more information. , amber amber, fossilized tree resin . Amber can vary in color from yellow to red to green and blue. The best commercial amber is transparent, but some varieties are cloudy.
..... Click the link for more information. , coral coral, small, sedentary marine animal, related to the sea anemone but characterized by a skeleton of horny or calcareous material. The skeleton itself is also called coral.
..... Click the link for more information. , and jet; of these, pearl is usually counted as a precious stone.

Counterfeit Gemstones

Artificial and imitation gems are of various kinds. Synthetic stones are made in the laboratory of the same chemical elements as natural stones. Among the synthetic gems produced commercially are rubies, sapphires, emeralds, and spinels. Diamonds of gem quality have also been manufactured. Color changes are produced in diamonds by exposing them to radioactive bombardment. Synthetic stones may sometimes be detected by the presence of air bubbles, which, when numerous, cause a cloudy appearance; by having curved rather than straight striae; and by their unnatural color. Doublets are made by combining a crown, or upper part, which is a thin slice of either the true stone or some inferior but hard gem, with a lower part of the true stone, a substitute stone, colored glass, or colored paste. Triplets generally consist of a layer of paste between two genuine stones of poor color. Paste (glass) gems usually contain lead and are consequently very soft; they soon lose their brilliance and color. Imitation pearls are glass or plastic beads coated with a preparation made from fish scales. A cultured pearl is made by inserting a small bead inside the oyster; the irritation causes the oyster to deposit pearly material upon the bead, leading to the formation of a pearl.

Bibliography

See J. Sinkankas, Gemstones of North America (1959); F. J. Sperisen, The Art of the Lapidary (rev. ed. 1961); R. Webster, Gems (2d ed. 1970); J. D. Dana, Manual of Mineralogy (18th ed., rev. by C. S. Hurlbut, Jr., 1971).

GEM, acronym for ground-effect machine

GEM: see air-cushion vehicle ground-effect machine or

Hovercraft. These vehicles are supported in various ways. Some of them have a specially designed wing that will lift them just off the surface over which they travel when they have reached a sufficient horizontal speed (the ground effect).
..... Click the link for more information. .

GEM

(1) (Graphics Environment Manager) A graphical user interface from Digital Research that was similar to the Mac/Windows environment. It was built into ROM in several Atari computers. The DOS version of Ventura Publisher came with a runtime version of GEM.

(2) (gem) The format for packaging Ruby programs via the RubyGems package manager. See Ruby.

gem [jem]

(mineralogy)

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

GEM

(mechanical engineering)

air-cushion vehicle

(operating system)

GEM - One of the first commercially available GUIs. Borrowing heavily from the Macintosh WIMP-style interface it was available for both the IBM compatible market (being packaged with Amstrad's original PC series) and more successfully for the Atari ST range. The PC version was produced by Digital Research (more famous for DR-DOS, their MS-DOS clone), and was not developed very far. The Atari version, however, continued to be developed until the early 1990s and the later versions supported 24-bit colour modes, full colour icons and a nice looking sculpted 3D interface.