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.

Corundum is relatively common and naturally transparent. In its purest form, it's colourless, but it changes colour with the presence of impurities. Examples include rubies and sapphires, which are gemstone varieties of corundum.

One of the hardest natural substances, corundum can scratch every mineral except diamond.

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.

Industrial Uses of Corundum

Because of its exceptional hardness, corundum has long been used as an industrial abrasive. In nature, it occurs either as crystalline corundum or as emery, a granular rock composed mainly of corundum mixed with other minerals, usually spinel or iron oxides, such as hematite.

Historically, emery was used for grinding and polishing, particularly before modern synthetic abrasives became widespread. Today, it has largely been replaced by synthetic corundum, which can be manufactured efficiently and provides consistent hardness and durability.

The term emery board comes from emery, reflecting its traditional use as a natural abrasive, although modern emery boards typically use synthetic materials.

The Greek island of Naxos, particularly around Cape Emeri, was historically one of the most important sources of emery. 

Synthetic corundum is widely used in sandpapers, grinding wheels, cutting tools and other wear-resistant components. Owing to its strength and resistance to heat, it is also used in industrial ceramics, watch bearings, optical components and protective windows.

Diamond remains the hardest usable material, but synthetic corundum is often preferred in industry because it is far cheaper, more predictable, easier to manufacture in large quantities, and sufficiently hard for most abrasive and wear applications.

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.