Corundum

Formula: Al2O3
Colour: Colourless, blue, red, pink, yellow, grey, golden-brown
Lustre: Adamantine, Vitreous, Pearly
Hardness: 9
Specific Gravity: 3.98 - 4.1
Crystal System: Trigonal
Name:  "corinvindum" in 1725 by John Woodward and derived from the Sanskrit, kuruvinda ("Ruby"). Richard Kirwan used the current spelling "corundum" in 1794. Known by many names in ancient times: adamant, sapphire, ruby, hyacinthos, asteria, etc.