SiO2

Quartz Quartz, n. [G. quarz.] (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. [1913 Webster]

Note: The crystalline varieties include: amethyst, violet; citrine and false topaz, pale yellow; rock crystal, transparent and colorless or nearly so; rose quartz, rosecolored; smoky quartz, smoky brown. The chief crypto-crystalline varieties are: agate, a chalcedony in layers or clouded with different colors, including the onyx and sardonyx; carnelian and sard, red or flesh-colored chalcedony; chalcedony, nearly white, and waxy in luster; chrysoprase, an apple-green chalcedony; flint, hornstone, basanite, or touchstone, brown to black in color and compact in texture; heliotrope, green dotted with red; jasper, opaque, red yellow, or brown, colored by iron or ferruginous clay; prase, translucent and dull leek-green. Quartz is an essential constituent of granite, and abounds in rocks of all ages. It forms the rocks quartzite (quartz rock) and sandstone, and makes most of the sand of the seashore. [1913 Webster]