When you first hear of corundum you may think you have never heard it before but corundum is actually the same mineral as such popular gemstones like ruby’s and sapphires. Corundum is the second hardest mineral known to the science community. It is only second to diamond. This strength could be attributed strong and short oxygen-aluminum bonds. Color is highly variable and can be any color to colorless. It had a vitreous to admantine luster. It has a hardness of nine.