Topaz

Formula: Al₂SiO₄(F,OH)₂

Origin of name: from Greek Topazion, a Red Sea Island often covered in mist

Crystal system: orthorhombic

Crystal class: 2/m 2/m 2/m

Twinning: rare on {010}

Unit cell: a = 8.394 Å, b = 8.792 Å, c = 4.649 Å

Colour: colourless, white, gray, bluish, greenish, pink, yellow-brown

Diaphaneity: transparent

Luster: vitreous

Habit: crystals short to long prismatic, may be highly modified, to large size, massive. coarse to fine granular, columnar

Hardness: 8

Specific gravity: 3.49 to 3.57

Cleavage: 1; {001} perfect

Tenacity: subconchoidal to uneven, brittle

Streak: colourless

Synonyms/varieties: imperial topaz (yellow to red-orange gem variety), pycnite (columnar massive variety)

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