quartz

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A very hard mineral composed of silica, SiO₂, found worldwide in many different types of rocks, including sandstone and granite. Varieties of quartz include agate, chalcedony, chert, flint, opal, and rock crystal.

Century Dictionary and Cyclopedia

1. n. The common form of native silica, Or the oxid of silicon (SiO₂). Silica is also found in nature in the minerals opal and tridymite (which see). Quartz occurs crystallized and massive, and in both states is most abundantly diffused, being one of the constituents of granite, gneiss, and many other crystalline rocks, forming quartzite and sandstone, and making up the mass of the sand of the sea-shore. When crystallized it commonly occurs in hexagonal prisms, terminated by hexagonal pyramids. It belongs, however, to the rhombohedral division of the hexagonal system, and its forms are sometimes very complex. Optically it is remarkable as exhibiting the phenomenon of circular polarization, the right- and left-handed character of the crystals optically corresponding to the arrangement of the modifying trapezohedral planes present. It scratches glass readily (hardness 7), gives fire with steel, becomes electrified by friction, and also by heating and pressure. It is infusible in the flame of the blowpipe, and insoluble in ordinary reagents except hydrofluoric acid. Its specific gravity is 2.66 when pure, and the luster vitreous or in some cases greasy to dull. The colors are various, as white or milky, gray, reddish, yellowish, or brownish, purple, blue, green. When colorless, or nearly so, and crystallized, it is known as rock-crystal (which see): here belong the “Lake George diamonds,” etc. Other distinctly crystalline varieties are the pink, called rose-quartz; the milk-white, milk-quartz; the purple or bluish-violet, amethyst; the smoky-yellow or brown, smoky quartz or Cairngorm stone, called morion when black or nearly so; the yellow, false topaz or citrine; the aventurin, spangled with scales of mica or hematite; sagenitic, containing acicular crystals of rutile; the cat's-eye, opalescent through the presence of asbestos fibers. The cryptocrystalline varieties are named according either to color or to structure: here belong chalcedony, agate in many forms, onyx, sardonyx, carnelian, heliotrope, prase, chrysoprase, flint, hornstone, jasper, basanite, agatized wood, etc. (see these words). The transparent varieties of quartz (amethyst, smoky quartz, etc.) are used for cheap jewelry, also when colorless for spectacles (then called pebble), and for optical instruments. Quartz prisms are useful in spectrum analysis, since quartz is highly transparent to the ultra-violet rays. (See spectrum.) Beautiful spheres of rock-crystal, sometimes several inches in diameter, occur in Japan. The massive colored kinds of quartz are much used as ornamental stones, especially the agates and agatized or fossil wood, onyx, etc. In these cases the colors are often produced or at least heightened by artificial means. Pulverized quartz is employed in making sandpaper; also when pure for glass-making, and in the manufacture of porcelain. Quartz-veins are often found in metamorphic rocks, and frequently contain rich deposits of gold; hence, in California and other gold-mining regions mining in the solid rock is commonly called quartz-mining, in contradistinction to placer and hydraulic mining. See cut under geode.

Wiktionary

1. n. mineralogy The most abundant mineral on the earth's surface, of chemical composition silicon dioxide, SiO2. It occurs in a variety of forms, both crystalline and amorphous. Found in every environment.

GNU Webster's 1913

1. n. (Min.) A form of silica, or silicon dioxide (SiO2), occurring in hexagonal crystals, which are commonly colorless and transparent, but sometimes also yellow, brown, purple, green, and of other colors; also in cryptocrystalline massive forms varying in color and degree of transparency, being sometimes opaque.

WordNet 3.0

1. n. colorless glass made of almost pure silica
2. n. a hard glossy mineral consisting of silicon dioxide in crystal form; present in most rocks (especially sandstone and granite); yellow sand is quartz with iron oxide impurities