Definition: Corundum is a family of minerals that provides us with rubies and sapphires. Corundum is durable, and is rated at a harness of 9 on the Mohs scale of Mineral Hardness (diamond, at 10, is the hardest mineral).
Rubies are red versions of corundum, but sapphires exist in many colors, including blue, pink, yellow, colorless and shades in-between.