Trivia

Rubies and sapphires are made of corundum -- the hardest known rock after the diamond.

Corundum is the second hardest natural mineral known to science. The hardest mineral, diamond is still four times harder than corundum. The hardness of corundum can be partially attributed to the strong and short oxygen-aluminum bonds. These bonds pull the oxygen and aluminum atoms close together, making the crystal not only hard but also quite dense for a mineral made up of two relatively light elements.