Gems : Corundum

Corundum is better known as two closely related varieties of gemstone: sapphire and ruby.
Corundum can be colourless, red, pink, blue, black, brown, orange, yellow, green, indigo, violet, or mauve. Red corundum, and most pink corundum, is called ruby, while all the other colours are called sapphire. Apart from blue, the colours are usually in the name, such as yellow sapphire. ‘Sapphire’ on its own indicates blue.
Pure corundum often called ‘white’, is actually colourless. Although quite rare, it tends not to be valuable because it’s not very brilliant.