gem oxides

Oxides and dioxides, another gem group, includes ruby, sapphire, chrysoberyl, spinel, hematite, opal, and the many varieties of quartz. The transparent gem form of corundum, which is fairly pure aluminum oxide (Al2O3), is called ruby when its color is a distinct scarlet or crimson red; found chiefly in Myanmar, Thailand, and Sri Lanka, it is classified among the most valuable of gems. In other colors, from pink through purple, blue, green, yellow, and orange, gem corundum is called sapphire. Synthetic rubies are manufactured by the fusion of pure aluminum oxide, with chromium oxide added as a coloring agent. Sapphires are found chiefly in Thailand, India, Sri Lanka, and Myanmar and in Australia and in the United States (in Montana and Idaho). Synthetic sapphires are made by the fusion of aluminum oxide, with titanium oxide added as a coloring agent. Vitreous, transparent to translucent gem forms of beryllium aluminum oxide (BeAl2O4) include the golden yellow chrysoberyl and the more valuable variety called cymophane, or cats-eye, as well as the rare alexandrite, remarkable in that it is green by daylight and raspberry red under artificial light. True cat's-eye, a variety of chrysoberyl from Sri Lanka and Brazil, displays a thin band of reflected light on its surface when cut as a cabochon; this optical effect, known as chatoyancy, is caused by the reflection of light from very thin, closely spaced filaments in parallel arrangement within the stone. Alexandrite, first discovered in Russia's Ural Mountains on the birthday of Czar Alexander II, for whom it was named, is now found chiefly in Sri Lanka and Brazil. Transparent red, blue, and green varieties of the mineral spinel, magnesium aluminum oxide (MgAl2O4) are used as gemstones. Myanmar and Sri Lanka are the principal producing countries, but much gem-quality spinel is now produced synthetically. The gem form of hematite, an oxide of iron (Fe2O3) containing about 70% metal, occurs in steel-gray to black crystalline forms. Extensive and richly productive deposits occur in the United States in the Lake Superior region (Michigan, Minnesota, and Wisconsin) and the Birmingham district (Alabama). Opal is a mineral consisting of poorly crystalline to amorphous silica, or hydrous silicon dioxide. (SiO29nH2O); the water content is quite variable but usually ranges from 3% to 10%. Precious, or gem, opal is richly iridescent. Specimens feature a remarkable play of changing colors, usually in red, green, and blue; this is due to a specific internal structure consisting of regularly packed uniform spheres of amorphous silica a few tenths of a micron in diameter. Sphere diameter and refractive index determine the range of colors displayed. White opal and black opal, named for the background color, are two of three main classes of gem opal; the color is due to fine-grained impurities. Fire opal is a bright red transparent or translucent opal that may or may not show a play of color. Most precious opal comes from the Coober Pedy and Andamooka fields in South Australia. The original source, known in Roman times, was in present eastern Slovakia. Precious opal has also been mined in Honduras, Mexico, and the Virgin Valley in Nevada. Quartz, or silicon dioxide (SiO2), is used extensively as a gem material. Occurring in crystals of the hexagonal system, the mineral commonly has the form of a six-sided prism terminating in a six-sided pyramid. It may be transparent, translucent, or opaque; it may be colorless or colored. Crystalline quartz varieties include ordinary colorless crystallized quartz, or rock crystal; rose quartz; yellow quartz, or citrine, sometimes used as imitation topaz; smoky quartz, or cairngorm stone; milk-white milky quartz; aventurine quartz, which contains scales of hematite or mica; and amethyst. Violet to purple in color, amethyst is the most highly valued of the semiprecious quartzes; Brazil, Uruguay, Sri Lanka, Siberia, and parts of North America are important sources of supply. The gem form of the so-called cryprocrystalline quartz, the crystal structure of which can be seen only under the microscope, if at all, is chalcedony. Translucent to transparent with a waxy luster, the name chalcedony is applied more specifically to white, gray, blue, and brown varieties. Some varieties, differing in color because of the presence of impurities, are agate, bloodstone, carnelian, chrysoprase, jasper, onyx, sard, and sardonyx. Carnelian's bright orange-red color is apparently caused by iron oxide; sard is brownish. Agates are colorful, banded rocks, identical in chemical structure to jasper, flint, chert, petrified wood, and tiger's-eye. Important sources of agate are Brazil, Uruguay, and the United States (Oregon, Washington, and around Lake Superior). The mineral is often found in association with opal. Onyx differs from agate only in that the bands of which it is composed are parallel and regular. Its appearance is most striking when the bands are of sharply contrasting colors; black and white specimens are often used for cameos. Sardonyx contains onyx and carnelian or sard. Cubic zirconia, a synthetic, single-crystal form of zirconium dioxide, is the best diamond simulant yet known; India is currently the world's largest user.