quartz, 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. It occurs in crystals of the hexagonal system, commonly having the form of a six-sided prism terminating in a six-sided pyramid; the crystals are often distorted and twins are common. Quartz may be transparent, translucent, or opaque; it may be colorless or colored. Varieties are classified as crystalline and cryptocrystalline. Crystalline varieties include ordinary colorless crystallized quartz, or rock crystal; rose quartz; yellow quartz, 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. Varieties of cryptocrystalline quartz, the crystal structure of which can be seen only under the microscope, if at all, are chalcedony, flint, hornstone, and chert. Colored varieties of chalcedony known by special names are carnelian, sard, chrysoprase, agate, onyx, sardonyx, and jasper. Clastic quartzes are sand and sandstone.