About the Mineral Corundum
Corundum is a relatively common natural mineral that's a crystalline form of aluminium oxide (a chemical compound of aluminium and oxygen) with traces of iron, titanium and chromium. Naturally transparent it changes colour with the presence of impurities but in its purest form is colourless. Corundum is one of the hardest natural minerals known to man and can scratch almost every other mineral on Earth. For this reason historically it has been widely used as an abrasive although in more recent years it has gradually been replaced with a synthetic substitute.
On Mohs scale of mineral hardness corundum grades 9 so it's softer than diamond which grades 10. Although only one grade higher diamond is four times as hard as corundum.
Rubies and sapphires both come from the mineral corundum but only red corundum is correctly known as ruby. All other colours are known as sapphire. With the presence of chromium corundum turns red but depending on the amount present, other impurities including iron and titanium can cause a variety of different colours including blue, yellow and black.
Red corundum - Photo; Stan Celestian. Click to enlarge
Inclusions of the mineral rutile in either red or blue corundum can result in an optical phenomenon known as asterism and when present, stones are better known as star ruby or star sapphire.