...surround some gems. As late as the 18th century many people used powdered gems to "prevent" or "cure" disease. They thought sapphire would relieve insanity, powdered jet would kill toothache, and topaz ease asthma. Even today some superstitious people wear amber beads to "cure" goitre; others fear that opals bring bad luck. Some famous gems are believed to carry a curse.

Gem Materials...

...Most gem stones are hard minerals. Their colours range from the colourless transparency of diamonds to pure black. Some are mottled or banded. Others reflect light waves in sparkling brilliance.

Gem minerals are usually crystal. Mineral crystals vary in size from huge blocks weighing many tons to tiny specks that can be seen only under a microscope. Rock crystal, a common gem material, is found in chunks large enough to be carved into massive vases and plates. The crystals of chalcedony are so tiny that this mineral was long thought to be noncrystalline.

Minerals such as chalcedony are called cryptocrystalline (Greek for "hidden crystal"). A very few gem materials, including opal and turquoise, seem to be amorphous (Greek for "without form"). A few gem materials are organic substances or have an animal or vegetable origin.

The pearl, for example, is formed from secretions of molluscs. The pearls of great value come only from the pearl oyster (see Pearl). Amber is a fossilised tree resin; jet, a compact form of lignite (brown coal); and coral, the skeletons of tiny sea animals. (See also Amber; Coral.)

Manufactured gems...

...early in the 19th century several experimenters successfully produced artificial reproductions of rare stones, such as the ruby. In 1902 Auguste Verneuil of Paris established the first commercially successful process for making such synthetic gems. The chief raw material is extremely pure, powdered aluminium. This is sifted through an intense oxyhydrogen flame, and the fused material drops down on a sticklike support.

A lump (boule) is gradually built up, cooled, and cut. Pure alumina yields white sapphire. Mixed with a small amount of chromium oxide, it makes synthetic ruby. Other mixtures yield blue, green, rose, or violet sapphires. A variation of the Verneuil process, developed in the United States in World War II, produces slender rods up to 30 inches long. They are especially useful in the manufacture of jewelled bearings for precision instruments. Synthetic gems are used principally in industry. They have replaced natural stones in the manufacture of most watches. Chemically, synthetic gems and natural gems are identical. An expert, however, can tell a synthetic gem by physical differences. The thin layers formed by the material as it builds into a boule may be seen under a microscope. Gas bubbles are sometimes found in synthetic gems, whereas natural gems may have bubbles of liquid.

Other manufactured types are imitation gems and doublets. For transparent imitation gems such as ruby, diamond, or sapphire glass is used. It is a very hard variety called paste, or strass giving the name "paste" to imitation gems.

A real gem can also be combined with less valuable material. A thin slice of ruby, for example, may be cemented to a base of red paste or of garnet. The ruby forms the top of the cut stone and the combination is a doublet.

Cutting & Polishing...

...the beauty of all gems is increased by cutting and polishing. In ancient times and in the Middle Ages, stones were merely smoothed and rounded. From this treatment developed the cabochon cut of today.

Stones cut in a cabochon style are oval or round in outline and have a polished, domed surface. The bottom is usually flat. In the late Middle Ages or early Renaissance, people discovered the art of cutting stones in a complex of flat surfaces, or facets. Earliest was the table cut.

This was a simple pattern with a rectangular principal face, or table, on top, surrounded by four sloping quadrilateral facets. The base facet, the culet, was much smaller than the table. The diamond's hardness makes it an extremely difficult gem to cut, and diamond cutting is a specialised craft in the gem-cutting industry.

Cutters of other gems (also called lapidaries) use a sandstone or carborundum grindstone. For faceted cuts the grindstone is flat surfaced; for cabochon cuts, it is grooved. While grinding, some lapidaries hold the gem in their fingers; others use a lapidary's stick (dop), to which the gem is cemented.

They polish the gems by holding them against a canvas-covered wheel, impregnated with jeweller's rouge, ferric oxide. Jewellery making is a popular hobby. Gem materials, cutting equipment, and courses in jewellery making are available in most large cities.

Gemmology...

...the science of gems is called gemmology. Gem stones were formerly classed as precious, semiprecious, and ornamental stones. Today this classification is largely replaced by the broader term gem materials.

The following list includes the major gem materials and some of the less important ones.

Achroite. A colourless variety of tourmaline.

Agate. A type of chalcedony, with coloured bands, first found on the banks of the river Achates; hence the name agate. The markings of moss agates occasionally resemble natural objects and so were much prized in the past. Most agates are naturally greyish and are artificially coloured. They are usually soaked in solutions of sugar or honey and acid or salt solutions and then heated.

Alexandrite. Named after Alexander II of Russia. A variety of Chrysoberyl, grass green in Color. Shows a red hue in artificial light.

Amazonite, or Amazonstone. A feldspar, jadelike in Color. Found in Canada, Brazil, Russia (Ural Mountains), Tanzania, Madagascar, Virginia, and Colorado.

Amethyst. A transparent violet-to-purple quartz. Believed by the ancient Greeks to prevent intoxication. Amethysts were used in both Greek and Roman jewellery. Under certain conditions the Color of some amethysts may be improved by heating. Under other conditions heating may turn them yellow, then clear. Found in quantity in southern Brazil and northern Uruguay. Smaller amounts come from India, Sri Lanka, and Madagascar.

Aquamarine. Transparent sea-blue or sea-green beryl; of the same family as the emerald but far less valuable. Found in many parts of the world, particularly Brazil, Sri Lanka, India, and Madagascar, and in Maine, New Hampshire, Connecticut, and North Carolina. An aquamarine crystal found in Brazil in 1910 weighed about 240 pounds (110 kilograms).

Aventurine. A quartz spangled with yellow mica or other mineral. Imitation Aventurine is called goldstone.

Azurite. An azure-blue copper carbonate found in most copper mines. Pliny called it caeruleum ("like the sky"). Used for colouring tabletops, vases, and jewellery. Found in Namibia, France, Chile, Zaire, Arizona, Utah, and Montana.

Beryl. A silicate occurring in several forms. Includes the emerald, aquamarine, golden beryl, Morganite, and goshenite.

Bloodstone, or heliotrope. A dark-green chalcedony (quartz) spotted with red jasper. Prized in the Middle Ages for carvings of martyrs, with the red flecks representing blood spots. Was called St. Stephen's Stone. Widely used in signet rings, cuff links, tiepins. Found in the Ural Mountains, India, and the western United States.

Cairngorm. A smoky yellow to smoky brown quartz, readily decolourised by heating. Very popular in Scotland, it is sometimes called Scotch topaz. Used for pins, brooches, and similar jewellery.

Carbuncle. A garnet cut en cabochon; that is, flat on the bottom, rounded above, without facets. In ancient times any red stone was called a carbuncle, and a mythical "carbuncle" was said to give out light in darkness.

Carnelian, or Sard. Name given to orange-red chalcedony. Used for ornamental carvings. Chief sources are Australia, India, and Brazil. Cat's-eye. The cheaper cat's-eye is of quartz, the more highly valued is of Chrysoberyl. Colours range from apple green to olive, from yellow to brown, but each Color has a streak or line through the middle, of varying brilliance, like a cat's eye. Found in Sri Lanka, India, Brazil, and Bavaria.

Chalcedony. A cryptocrystalline quartz, used by gem engravers since ancient times. Bible has many references to it. Color usually white, pale blue, or gray. Varieties include carnelian (or Sard), Chrysoprase, Prase, plasma, bloodstone (or heliotrope), agate, onyx, Sardonyx, jasper. Used chiefly for ornamental carvings. Widely distributed. Excellent stones are obtained from the Ural Mountains, Uruguay, and Romania.

Chrysoberyl. A rare, unusually hard stone. One variety, the Alexandrite, green by day and red by lamplight, had a great vogue among aristocrats of czarist Russia. Other varieties include Chrysolite and cat's-eye.

Chrysolite. An olive-green vitreous magnesium iron silicate, also called olivine and peridot. It is crystalline and, when transparent, used as a gem.

Chrysoprase. An apple-green type of chalcedony. Citrine. A clear yellow quartz resembling topaz. Used for pendants, bracelets, earrings, and pins. Coral. Precious coral is red and branching, found in the Mediterranean. Black coral has been found in the Persian Gulf and on the Great Barrier Reef off Australia (see Coral).

Diamond. Pure crystallised carbon, the most highly esteemed of all gem stones (see Diamond).

Emerald. A deep-green brilliant emerald is one of the costliest of gems. The emerald is a variety of beryl. The finest stones come from Colombia. Other sources are Brazil, Egypt, Australia, Austria, Norway, and North Carolina.

Feldspar. Silicate feldspar produces several gem stones: Amazonite, a bright green; moonstone, opalescent; Sunstone, reddish; Labradorite, gray with play of colours in blue and green and sometimes yellow or red. Feldspars are widely distributed.

Garnet. A gem, usually deep red, of several varieties, including the almandite and Pyrope. When cut en cabochon garnets are called carbuncles. Garnet is found in nearly all colours but blue. Rarest of garnets is the green demantoid, found only in the Ural Mountains. Others come from Brazil, India, Sri Lanka, and the United States.

Girasol. A variety of opal. Iridescent. Heliotrope. Same as bloodstone.

Hematite. Black crystalline iron; when finely divided, streaked with red. Popular for costume jewellery. Found in England, Norway, Sweden, island of Elba, and Lake Superior region.

Hyacinth, or jacinth. Variety of zircon.

Jade. A name applied to jadeite and nephrite. The Color is usually green but sometimes a whitish cast appears. The stone takes a high polish. Jewellery, cups, and bells are made of it, and poems of Chinese emperors have been carved in priceless jade bowls. In China jade is considered the most precious gem stone.

Jasper. An opaque ornamental chalcedony, in red, yellow, brown, green, or gray-blue, used for vases, tables, mantels, and pillars. Jasper is often striped. Swiss or German lapis is jasper artificially coloured.

Jet. A hard, black lignite (variety of coal). Takes high polish. Popular for ornaments and costume jewellery. Many jet articles manufactured in Yorkshire, England, from local lignite and imports from Spain. Pennsylvania anthracite and Scottish cannel coal are often used in place of jet.

Labradorite. A pearl-gray or brown feldspar, showing many colours in light. Takes high polish.

Lapis lazuli. The blue mineral lazurite. Mines in Afghanistan, worked for 6,000 years, probably world's oldest mines. Called "sapphire" by ancient world. Also found in Chile, Siberia, and southern California. Used for jewellery, mosaics, inlaid work, vases.

Malachite. A green copper carbonate; used for making ornaments. Marcasite. Differs from pyrite only in crystal structure; often set as brilliants in costume jewellery.

Moonstone. A milky, translucent feldspar. The finest specimens are found in Sri Lanka.

Morganite. A rose-coloured beryl. Named for J. Pierpont Morgan.

Obsidian. A smoky natural glass of volcanic formation, abundant in Yellowstone National Park.

Olivine. A silicate of iron and magnesium. Bottlegreen variety best; others are peridot and Chrysolite.

Onyx. The cameo carver's favourite material, chalcedony, with horizontal stripes of black and white. Also used for tabletops, lamps, and decorations.

Opal. noncrystalline. silica, often combining several colours and frequently opalescent.

Pearl. Not a true gem stone.

Peridot. A yellowish green olivine. Found in St. John's Island (Red Sea), Australia, Arizona, Hawaii, and, rarely, other places.

Plasma. A variety of green chalcedony.

Prase. A dull-green chalcedony. Precious serpentine. Translucent and rich green.

Pyrite. A pale yellow opaque iron disulphide.

Pyrope. A variety of garnet, often deep red. Quartz. Crystalline silica, the most common of all minerals. Gem varieties include rock crystal, amethyst, citrine, siderite, Aventurine, chalcedony, cat's-eye, and rose, smoky, milky, and rutilated quartz.

Rhinestone, or brilliant. Imitation gem made from special kinds of glass called paste, or strass. Colourless, sparkling, in imitation of diamonds. Rhinestone glass made from heating mixture of quartz, red lead, potassium carbonate, borax, white arsenic.

Rhodolite. A pink or purple variety of Pyrope.

Rock crystal. A clear quartz much used for beads, vases, goblets, and crystal balls.

Ruby. A transparent red corundum valued according to shade of Color. Large rubies are often worth more than fine diamonds of the same size. Pigeon-blood (deep carmine-red) rubies, which seldom exceed three carats, are obtained from Myanmar. Darker rubies come from Thailand. Rubies also occur in Sri Lanka, Afghanistan, and North Carolina.

Sapphire. A transparent corundum, or aluminium oxide. Sapphires range from white through blue, violet, yellow, and green to near black. Blue star sapphires with six rays are favourite stones. Fine sapphires are equal in value to diamonds of equal size. Found in Myanmar, Thailand, Sri Lanka, and Jammu and Kashmir.

Sardonyx. A variety of onyx with parallel layers of white, brown, black, and red Sard. Used in cameos and intaglios. Chief sources are Brazil, Uruguay, Madagascar, India, and Texas.

Spinel. A red, violet, blue, green, or yellow magnesium aluminate. Sometimes called "mother of ruby." Found in Myanmar, Thailand, and Sri Lanka.

Sunstone. A reddish feldspar with fiery reflections. Tiger-eye. Like cat's-eye, a chatoyant stone; that is, when polished, the interior shows an undulating light.

Topaz. An aluminium fluosilicate occurring in tawny yellow, blue, green, reddish violet, pink, and colourless varieties. Found in Brazil, Siberia, Sri Lanka, and the United States.

Tourmaline. A complex aluminium borosilicate occurring in great variety colourless, rose red, green, blue, yellowish, green, honey yellow, violet, and dark blue. Most tourmaline is obtained from Brazil, Elba, Madagascar, and Maine, Connecticut, and California.

Turquoise. A hydrous copper aluminium phosphate ranging from sky blue to greenish gray. Often cut en cabochon. Widely used for bracelets, earrings, pins, belt buckles, and saddle studdings. Faded by heat and sunlight. Found in Iran, New Mexico, and Nevada.

Zircon. A zirconium silicate usually occurring in brownish, gray, or brownish red varieties, but sometimes in yellows and greens. Colourless and blue varieties are usually produced by heating brown zircons. Zircons come chiefly from Sri Lanka and Indochina.