A ruby, whose name is from the Latin for red, is a pink to red gemstone. It is a special variety of the very hard mineral known as corundum, and has a hardness of 9. This is just below diamond (which is 10) and so rubies made artificially can be used as an abrasive. Rubies, like all corundum, are an oxide of aluminium. The red colour is because there is contamination by the element chromium. If the corundum is coloured green, the mineral is called sapphire. Ruby is a precious stone.