Sapphire : as a mineral

General
Category	Mineral Variety
Chemical formula	aluminium oxide, Al2O3
Identification
Color	Every color except red (which is ruby)
Crystal habit	massive and granular
Crystal system	Trigonal
Cleavage	None
Fracture	Conchoidal, splintery
Mohs Scale hardness	9.0
Luster	Vitreous
Refractive index	1.762-1.778
Pleochroism	Strong
Streak	White
Specific gravity	3.95-4.03
Fusibility	infusible
Solubility	insoluble
Other Characteristics	Coefficient of thermal expansion 5e-6–6.6e-6/K

Sapphire refers to gem varieties of the mineral corundum, an aluminium oxide (Al2O3), when it is a color other than red. It can be found naturally or manufactured in large crystal boules for varied applications, including infrared optical components, watch faces, high-durability windows, and wafers for the deposition of semiconductors such as GaN nanorods.

The mineral corundum consists of pure aluminium oxide. Trace amounts of other elements such as iron, titanium and chromium give corundum their blue, yellow, pink, purple, orange or greenish color. Sapphire includes any gemstone quality varieties of the mineral corundum except the fully saturated red variety, which is instead known as ruby, and the pinkish-orange variety known as padparadscha.

source :wikipedia,the free encyclopedy
more information at : http://en.wikipedia.org/wiki/Sapphire