Corundum ( Sapphire )
Structure : TrigonalHardness : 9
Specific Gravity : 4
Refractive index : 1.76 - 1.7
Doubly Refracting (bi-refraction) : 0.008
There are two different varieties of Corundum as far as gemstones are concerned, Ruby and Sapphire. I wont go into the extensive origins, colors and crystalline patterns that make up sapphire. For that please see this Wikipedia article.
For more gemstones on our website that are for sale: