Corundum Properties, Facts and Photos
Corundum Hardest Mineral After Diamond
Corundum 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 it can scratch every mineral except diamond. For this reason it has long been used as an abrasive.
In recent years the use of corundum as an abrasive has gradually been replaced with synthetic substitutes.
On 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. 
With the presence of chromium corundum turns red. Depending on how much is present other impurities including iron and titanium cause different colours including blue, yellow and black.
Red corundum is known as ruby whilst 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.
To be fully appreciated the stone must be cut as a cabochon.
Article Pictures
The red corundum at the top of our article is on display in the Natural History Museum Los Angeles.
The stones in the second photo are rubies and sapphires. In the lower part of the split image they're being illuminated by UV light.
Both photos are clickable and redirect to the original images. Photos courtesy of Stan Celestian.
