Corundum is a crystalline form of aluminum oxide and is one of the hardest minerals on Earth, ranking 9 on the Mohs scale of hardness. The variety of corundum known as sapphire refers specifically to the blue or colored forms of corundum, which have been highly valued as gemstones for centuries. While blue is the most well-known color, sapphires come in a wide range of hues, including pink, yellow, green, and even colorless, depending on the trace elements present in the crystal structure.

When Corundum is red - we call that Ruby.