quartz

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quartz (rock crystal) Widely distributed rock-forming silicate mineral SiO2 with the related semi-precious varieties rose quartz (pink), amethyst (purple), cairngorm (dark brown and similar to smoky quartz, although the original cairngorms from the Scottish mountain range were of topaz), citrine (light brown); sp. gr. 2.65; hardness 7; trigonal; commonly colourless or white, but can occur in a variety of colours; vitreous lustre; crystals usually six-sided prisms terminated by six-faced pyramids, the prisms often striated, also occurs extensively in massive form; no cleavage; conchoidal fracture; found in many igneous and metamorphic rocks, extensively in clastic rocks, and a common gangue mineral in mineral veins. See also AGATE; CHALCEDONY; COESITE; CRISTOBALITE; FLINT; JASPER; ONYX; OPAL; STISHOVITE; and TRIDYMITE.

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