Quartz

Second only to feldspar in crustal abundance, quartz is a silicon dioxide (SiO2) that occurs in virtually every geological environment and forms a component of nearly every rock type. It crystallizes in the trigonal system, commonly as prismatic, massive, or druzy forms, and shows a vitreous luster, a white streak, no cleavage, and a conchoidal fracture. With a Mohs hardness of 7, it defines that point on the scale. Its varieties are numerous, including amethyst, citrine, rose quartz, smoky quartz, and the microcrystalline chalcedony, and its specific gravity is a steady 2.65. Because it is both hard and chemically resistant, quartz survives weathering to become the dominant mineral of many sands.

Quartz is the second most abundant mineral in Earth's continental crust, after feldspar. It occurs in virtually every geological environment and is a component of almost every rock type. Varieties include amethyst, citrine, rose quartz, smoky quartz, and chalcedony.