quartz

A hard mineral that is a crystalline form of silica (silicon dioxide, SiO₂); it is colorless when pure ("rock crystal"). Quartz is a constituent of many rocks, especially igneous rocks such as granite and quartzite, metamorphic rocks such as gneisses and schists, and sedimentary rocks such as sandstone and limestone. It is the second most abundant mineral on Earth after feldspar.

Quartz crystallizes in the trigonal system. A well-formed quartz crystal consists of a six-sided prism that terminates in six-sided pyramids. Quartz is usually colorless and transparent, in which form it is called rock crystal. Colored varieties,a number of which are used as gemstones, include amethyst, citrine (yellow), rose quartz (pink), milk quartz (white), smoky quartz (brown), agate, jasper, onyx, carnelian, and chalcedony. It has a hardness of 7 on Mohs scale; relative denisty 2.65.

A quartz crystal produces electricity through the process of piezo-electricity (see piezo-electric effect) when a mechanical stress is applied to it. This has led to the use of quartz for making oscillators for clocks, radios, and radar instruments. Quartz is also used in optical instruments and in glass, glaze, and abrasives.

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• GEOLOGY AND PLANETARY SCIENCE

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