Sapphire is the most precious and valuable blue gemstone. It is a very desirable gemstone due to its excellent color, hardness, durability, and luster. In the gem trade, Sapphire without any color prefix refers to the blue variety of the mineral Corundum

Color often blue, but varies
Mohs scale hardness 9.0
Specific gravity 3.95 – 4.03
Refractive index 1.768 – 1.772
Pleochroism Strong