Second most abundant mineral (after feldspar) in the Earth's crust, present in many rocks. Quartz, which consists of silica, or silicon dioxide (SiO), 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. Crushed quartz is used as an abrasive in sandpaper; silica sand is employed in sandblasting; and sandstone is used whole to make whetstones, millstones, and grindstones. Silica glass (or fused quartz) is used in optics to transmit ultraviolet light. Tubing and various vessels of fused quartz have important laboratory applications, and quartz fibres are employed in extremely sensitive weighing devices.

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