Corundum

Corundum is a mineral. It is very hard, with a Mohs hardness of 9 (diamond is 10). It is usually clear. Its chemical formula is mostly aluminium oxide, .

Different metal atoms can replace some of the aluminium atoms in corundum. These impurities give corundum different colours. Different colours of corundum have different names as gemstones. Rubies are corundum coloured red by chromium impurities. Sapphires are coloured by iron and titanium impurities, and take on many different colours depending on the amount of each element.

 Related pages 
Bauxite
Aluminium
Chromium(III) oxide

References