Corundum (which is an Asian word for ruby) is made mostly of aluminium oxide. It is produced deep inside mountain ranges as they form, and has developed as a result of great heat and pressure. It is often found near to magma chambers.

It is naturally transparent but becomes coloured when impurities are added. These transparent coloured forms of corundum are called ruby if red and sapphire if green or blue.

Corundum's main usefulness is because it is so hard. On Moh's scale of hardness it is 9 (diamond is 10), so it can be used to grind down almost all other materials. It is the 'grit' on most abrasive papers and grinding wheels. It is often called 'emery' if it is a mixture of corundum and other minerals.