Corundum

Formula:  Al₂O₃

Origin of name:  From an old East Indian name for the mineral

Crystal system:  Hexagonal

Crystal class:  -3 2/m

Twinning:  Common on {10-11}, lamellar; rare on {0001}

Unit cell: a = 4.76 Å, c = 12.99 Å

Colour:  Colourlss, gray, brown, blue, red, green, yellow, orange, purple

Diaphaneity:

Luster:  Vitreous to adamantine

Habit:  Crystals often well developed, steep-pyramidal, prismatic, tabular; barrel-shaped; massive

Hardness:  9

Specific gravity:  4.0 to 4.1

Cleavage:  2; {0001} and {10-11} parting

Tenacity:

Streak:

Synonyms/varieties:  Ruby (red gem var.), sapphire (non-red gem var.), emery (impure var.)

Comments:  Sometimes fluoresces under SW and LW