Corundum is a very hard mineral - Mohs Hardness Scale of 9, second only to a diamond. When comparing absolute hardness, a diamond is four times harder than corundum. A clear and colorless synthetic corundum is often used for watch crystals to resist scratching as it can only be scratched by another corundum surface or a diamond.
Corundum is an aluminum oxide. In perfect form, corundum is clear though various impurities will change the color of the mineral. When colored red, corundum is called a ruby. Other clear and translucent forms of corundum are called sapphires.