Corundum Properties, Facts and Photos
What is Corundum?
Corundum, which is the hardest mineral after diamond, is a crystalline form of aluminium oxide with traces of iron, titanium and chromium. Aluminium oxide is a chemical compound of aluminium and oxygen.
Rubies and sapphires are gemstone varieties of the mineral corundum.
Relatively common and naturally transparent, corundum changes colour with the presence of impurities. In its purest form, it's colourless.
One of the hardest natural substances, corundum can scratch every mineral except diamond. For this reason, it has long been used as an abrasive. In recent years, it has gradually been replaced with synthetic substitutes.
On the Mohs scale of hardness corundum grades 9. Diamond, which is the hardest mineral, grades 10. Although only one grade higher, diamond is four times as hard as corundum. 
With the presence of chromium, corundum turns red. Depending on the amount present, other impurities such as iron and titanium cause different colours including blue, yellow and black.
Red corundum is known as ruby, while all other colours are sapphire.
The optical phenomenon known as asterism can often be seen when inclusions of rutile are present. These stones are known as star ruby or star sapphire. The effect is best appreciated when stones are cut as a cabochon.
Article Pictures
The red corundum at the top of our article is displayed in the Natural History Museum of Los Angeles.
The stones in the second photo are rubies and sapphires. The lower image is the same stones when illuminated by UV light.
The photos are clickable and redirect to the original image. Photos courtesy of Stan Celestian.
