Corundum

Corundum is an attractive form of aluminium oxide used for jewellery. Other forms with the same composition and crystalline structure include ruby (red from traces of chrome) and sapphire (various colours, including blue and bright red, from iron or titanium).

Because of its hardness, large pieces of corundum are used in industry to make abrasives (sandpaper, cutting discs, etc.). It is also used for sandblasting. It can cause silicosis.