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see air-cushion vehicleair-cushion vehicle
(ACV), craft designed to travel close to but above ground or water. It is also called a ground-effect machine or Hovercraft. These vehicles are supported in various ways.
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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 monthmonth,
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. For the computation of the month and its harmony with the solar calendar and for the months in others than the
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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 diamonddiamond,
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
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; some forms of corundumcorundum
, mineral, aluminum oxide, Al2O3. The clear varieties are used as gems and the opaque as abrasive materials. Corundum occurs in crystals of the hexagonal system and in masses. It is transparent to opaque and has a vitreous to adamantine luster.
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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.
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, sapphiresapphire,
precious stone. A transparent blue corundum, it is classified among the most valuable of gems. Sapphires are found chiefly in Madagascar, Sri Lanka, and Myanmar and also in Thailand, Tanzania, Australia, Cambodia, and in the United States (in Montana).
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, Oriental emerald, Oriental topaz, and Oriental amethyst); and emeraldemerald,
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.
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. The chief semiprecious stones are aquamarineaquamarine
[Lat.,=seawater], transparent beryl with a blue or bluish-green color. Sources of the gems include Brazil, Siberia, the Union of Myanmar, Madagascar, and parts of the United States. Oriental aquamarine is a transparent crystalline corundum with a bluish tinge.
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, amethystamethyst
[Gr.,=non-drunkenness], variety of quartz, violet to purple in color, used as a gem. It is the most highly valued of the semiprecious quartzes. It is associated with a number of superstitions, being regarded as a love charm, as a potent influence in improving sleep, and
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, topaztopaz
, aluminum silicate mineral with either hydroxyl radicals or fluorine, Al2SiO4(F,OH)2, used as a gem. It is commonly colorless or some shade of pale yellow to wine-yellow; pale blue and pale green also occur, but natural red stones are
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, garnetgarnet,
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). The garnets are double silicates; one of the metallic elements is calcium, magnesium, ferrous iron, or manganese and the
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, tourmalinetourmaline
, complex borosilicate mineral with varying amounts of aluminum, iron, magnesium, sodium, lithium, potassium, and sometimes other elements, used as a gem. It occurs in prismatic crystals, commonly three-sided, six-sided, or nine-sided, and striated vertically.
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, spinelspinel,
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 limestones.
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, peridot (see olivineolivine
, an iron-magnesium silicate mineral, (Mg,Fe)2SiO4, crystallizing in the orthorhombic system. It is a common constituent of magnesium-rich, silica-poor igneous rocks; metamorphism of some high magnesium sediments also can form olivine.
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), zircon (see zirconiumzirconium
, metallic chemical element; symbol Zr; at. no. 40; at. wt. 91.224; m.p. about 1,852°C;; b.p. 4,377°C;; sp. gr. 6.5 at 20°C;; valence +2, +3, or +4.

Zirconium is a very strong, malleable, ductile, lustrous silver-gray metal.
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), chrysoberylchrysoberyl
[Gr.,=golden beryl], a beryllium aluminate used as a gem. It has a vitreous luster and is transparent to translucent. The more valuable cat's-eye is a variety of chrysoberyl. Another variety, alexandrite, was first discovered in the Ural Mts.
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, quartzquartz,
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.
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, opalopal
, a mineral consisting of poorly crystalline to amorphous silica, SiO2·nH2O; the water content is quite variable but usually ranges from 3% to 10%.
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, turquoiseturquoise,
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.
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, moonstonemoonstone,
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.
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, and jadejade,
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.
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. The organic gems are pearlpearl,
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 object—usually a
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, amberamber,
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. To be called amber, the resin must be several million years old; recently hardened resins are called copals.
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, coralcoral,
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.
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, 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.


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).


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


(mechanical engineering)


(operating system)
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.


(1) See JEM and GEMS.

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

(3) (Graphics Environment Manager) A early 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, enabling graphics-based page layout on a PC, which was not common at the time.