Ever heard the words “I would like a corundum ring please”? Probably not. Maybe because when corundum found in gem quality it is often called sapphire or ruby.

Ruby and Sapphire Image from - http://www.jewelry-secrets.com/
Ruby and Sapphire Image from – http://www.jewelry-secrets.com/
Colourations, crystal habit and hardness

Corundum is an aluminium oxide (Al2O3) and in its pure form is transparent. The impurities in the mineral give it the spectacular colourations; iron(III) creates yellow or green sapphires, titanium(III) gives pink colourations, chromium makes rubies (red) and titanium (IV) and iron (II) give sapphires (blue). [1]

Occasionally sapphire contain both yellow and blue and these are known as parti sapphires.

It grows naturally hexagonal crystals (fig 1.). There hardness is a 9  and is a defining minerals on the Mohs hardness scale which makes them the 3rd hardest mineral known to science.

Formation and Deposits

Corundum is formed in igneous such as; syenite, nepheline syenite and pegmatite and metamorphic rocks such as; schist, gneiss or marble, with marble usually containing the higher quality gems. However, when fossicking for rubies and sapphires you generally look in alluvial deposits as these gems are so hard they resist weathering and get plucked out of their host rocks and washed into streams.

Ruby in metamorphic rock - photo courtesy of True North Gems
Fig 1 – Ruby in metamorphic rock – photo courtesy of True North Gems

Good quality blue sapphires can range from US$1000-$3000 per carat. Rubies are more expensive as they are rarely good enough quality to facet, with deep red stones selling for over US$10,000 per carat.

Even in you do not have ruby of sapphire jewellery you may have corundum in your house as Emery boards and sandpaper are often made from grains of (synthetic) corundum. Synthetic corundum is now making its way into the jewellery market as well as the demand for natural flawless rubies and sapphires far outweighs the supply.

Cheeky gemstone tricks

As deep colours and clarity are desirable for jewellery, many of the stones cut for this purpose have been heat treated to intensify the colour. The corundum must be heated to >1400 C which removed the “silk” and clarifies the stones. This silk is often inclusions of rutile (TiO2).

Some lower grade sapphire can be artificially coloured using heat treatments in the presence of beryllium which diffuses into the sapphire, colouring it yellow or orange.

Where can you find them?

In Australia, Queensland has many gem fields were you can find your own sapphires ( Anakie, Rubyvale, Sapphire, Glenalva, The Willows, Inverell, Glen Innes). Sapphires are quite common throughout the world however rubies are primarily sourced from Myanmar (Burma).

If you are interested in fossicking you should contact your local fossicking or lapidary club. 

What’s you’re favourite gemstone?

[1] Rock-Forming Minerals: Non-Silicates: Oxides, Hydroxides and Sulphides edited by J. F. W. Bowles, R. A. Howie, D. J. Vaughan, J. Zussman