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

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

A hard mineral composed of pure silica; the most common of solid minerals, found in many kinds of rock, often in the form of crystals.

A hard mineral, common in rocks, as in granite.

n. [German, French] Pure silex, crystallized in masses of various colours, more or less transparent, and sometimes opaque.