Quartz

Quartz, rhombohedral form of silica, usually forming hexagonal prisms, colorless when pure. A common mineral (SiO₂), it is the chief constituent of sand, sandstone, quartzite, and flint and an essential constituent of high-silica igneous rocks, such as granite, rhyolite, and pegmatite. It also occurs as the gems chalcedony, agate, jasper, and onyx. Quartz is piezoelectric and is used to make oscillators for clocks, radio, and radar and to make windows for optical instruments. Crude quartz is used to make glass, glazes, and abrasives, and as a flux.