quartz, Second most abundant mineral (after feldspar) in the Earth’s crust, present in many rocks. Quartz, which consists of silica, or silicon dioxide (SiO2), 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 (or silica sand) are used in the manufacture of glass and ceramics and for molds in metal casting.