Corundum is an aluminum oxide with a chemical composition of Al2O3. The corundum mixed some chromium metal inside with a deep red color is known as a “ruby.” Gemstone-quality corundum with blue color is called a “sapphire.” Colorless corundum is known as “white sapphire.” Pure corundum is defined to have 9.0 on the Mohs scale. It is a hexagonal crystal structure with 4.00 proportion. Natural and synthetic corundum is used in a wide variety of fields because of their toughness, hardness, and chemical stability.