☞ 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.