Sapphire 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. However, the term Sapphire encompasses all other gem varieties and colors of Corundum as well, excluding Ruby, the red variety of Corundum, which has its own name since antiquity.