Chemical composition: ALSi04 (F, 0H)2 A1203 = 56.6%, Si02 = 33.4% and H20 = 10%.
(i) Form: Prismatic, columnar and granular.
(ii) Colour: Colourless, wine-yellow, straw-yellow, white, grayish, blue or pink.
(iii) Lustre: Vitreous.
(iv) Streak: White.
(v) Hardness: 8.
(vi) Cleavage: Perfect 1 set.
(vii) Fracture: Subconchoidal to uneven.
(viii) Specific gravity: 3.5 – 3.6.
(ix) Diaphaneity: Transparent to translucent.
Use: Topaz is used as abrasive and the transparent varieties are used as gemstone.