Quartz, the anhydrous mineral form of silica or silicon-dioxide crystallising in the Hexagonal system. It is sometimes extended to all forms of silica, including the non-crystalline chalcedony (q.v.), and jasper (q.v.), and even the hydrous opal (q.v.). The crystalline variety is 7 in the scale of hardness, and has a specific gravity of 2.6. Its most common crystalline form is the hexagonal prism terminated by six-sided pyramids. When colourless and transparent, it is known as rock-crystal, Brazilian pebble, Irish diamond, etc.; when violet, as Scotch amethyst; when smoky, as cairngorm. It also occurs rose-coloured; or milky, from the numerous cavities containing salt-water or liquid carbon-dioxide. All crystalline varieties break with a conchoidal fracture. It is one of the most abundant minerals, being an essential constituent of granite, eurite, felsite, liparite, gneiss, and mica-schist, and forming almost the whole of quartzite, sand, and sandstone.