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 density 2.65.
A quartz crystal produces electricity through the process of piezoelectricity (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.
Related category GEOLOGY AND PLANETARY SCIENCE
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