|Chemical formula||aluminium oxide, Al2O3|
|Color||Every color except red (which is ruby)|
|Crystal habit||massive and granular|
|Mohs Scale hardness||9.0|
|Other Characteristics||Coefficient of thermal expansion 5e-6–6.6e-6/K|
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