A hard mineral of many varieties that consists primarily of silica or silicon dioxide. Quartz has been important from the earliest times; crystals of it were known to the ancient Greeks as 'krystallos'. It is typically colorless to white, with some minor impurities which make it into many different colors. Quartz has great economic importance. Many varieties are gemstones, including amethyst, citrine, smoky quartz, and rose quartz. Sandstone, composed mainly of quartz, is an important building stone. Large amounts of quartz sand are used in the manufacture of glass and porcelain and for foundry molds in metal casting. Quartz is the second most abundant mineral in the Earth's crust after feldspar.