<chemical>

A form of silica, or silicon dioxide (SiO2), occurring in hexagonal crystals, which are commonly colourless and transparent, but sometimes also yellow, brown, purple, green, and of other colours; also in cryptocrystalline massive forms varying in colour and degree of transparency, being sometimes opaque.

The crystalline varieties include: amethyst, violet; citrine and false topaz, pale yellow; rock crystal, transparent and colourless or nearly so; rose quartz, rosecoloured; smoky quartz, smoky brown. The chief crypto-crystalline varieties are: agate, a chalcedony in layers or clouded with different colours, including the onyx and sardonyx; carnelian and sard, red or flesh-coloured chalcedony; chalcedony, nearly white, and waxy in luster; chrysoprase, an apple-green chalcedony; flint, hornstone, basanite, or touchstone, brown to black in colour and compact in texture; heliotrope, green dotted with red; jasper, opaque, red yellow, or brown, coloured by iron or ferruginous clay; prase, translucent and dull leek-green. Quartz is an essential constituent of granite, and abounds in rocks of all ages. It forms the rocks quartzite (quartz rock) and sandstone, and makes most of the sand of the seashore.

Origin: G. Quarz.

(01 Mar 1998)

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