Quartz is the mineral with the chemical formula SiO₂ that includes amethyst and citrine varieties.
Pure quartz is usually colorless, but trace elements, structural defects, and radiation can create different colors. Purple quartz is called amethyst, while yellow or orange quartz is commonly called citrine. Much commercial citrine is heat-treated amethyst, although naturally yellow quartz also occurs.
Quartz is widespread in igneous, metamorphic, and sedimentary rocks. Its resistance to chemical weathering allows quartz grains to survive transport and accumulate in sandstone and many beach sands. Quartz also has piezoelectric properties: pressure on a crystal can produce an electrical charge, which has made quartz useful in clocks, oscillators, sensors, and electronics.