Corundum is composed of aluminum oxygen. In the purest form it is aluminium oxide (A1, 03; A1 = 52.91
O = 47.1%). However, minor amounts of Cr, Ti, Fe+2, Ca, Mg, Ni, Si
Mn replaces Al.
(i) Form: Massive, coarse or fine granular crystalline; crystal varieties are barrel shaped pyramidal.
(ii) Colour: Common varieties are colourless, white, gray, greenis reddish. Gem varieties: ruby (with trace amount of Cr) is red, sap (with trace amounts of Ti, Fe+2) is blue, oriental amethyst is pu oriental emerald is green, oriental topaz is yellow.
(iii) Lustre: Adamantine to vitreous; crystal faces are usually dull.
(iv) Streak: White.
(v) Hardness: 9.
(vi) Cleavage: Absent but rectangular partings are common.
(vii) Fracture: Conchoidal or uneven.
(viii) Specific gravity: 3.9-4.1.
(ix) Diaphaneity: Transparent to translucent.
Corundum is used as an abrasive commonly known as emery, which is either ground from the pure or impure massive mineral. Gem varieties of corundum are ruby, sapphire, oriental amethyst, oriental emerald and oriental topaz coloured as red, blue, purple, green and yellow respectively.