Corundum

Chemical Formula : Al₂O₃
Hardness : 9
Cleavage : None, but common parting on twinned specimens.
Luster : Sub-metallic-splendent to vitreous
Color : White to clear when pure; with impurities any color is possible including brown and black.
Streak : White
Density : 3.98 when pure, otherwise 4.0-4.1
Diagnostic Festures and Trivia : Corundum is the definition of a hardness of nine on Moh's Hardness Scale. Corundum is the mineral name of the gemstones sapphire and ruby. All quality specimens of all colors are called sapphire unless a deep red and are rubies. These variations in color are due to various levels of impurites of titanium and iron in sapphires, and chromium in rubies. When these gems contain small inclusions of Rutile, then cat's eye and star specimens are formed. Dark, opaque corundum is also called emery, and is commonly used as an abrasive.
Crystal System and Habit : Hexagonal (trigonal); commonly tabular {0001} to short prismatic, also steep dipyramidal.
Twinning : Common on {01-12} and {0001} creating stacks like books.
Polymorphs : None
Index of Refraction : omega=1.767-1.772; epsilon=1.759-1.762