Corundum (Sapphire and Ruby)

Things to know about corundum (sapphire and ruby)


From a mineralogical point of view, ruby is a variety of corundum, an aluminium oxide to which a small amount of chromium is added to give it its coveted and ever-popular ruby red colour. However, it is not only chromium that gives colour to the otherwise colourless corundum, but also other colouring elements such as iron, titanium and others. This is called sapphire, which includes all colours except red.

Because of their attractive colour, sapphires and rubies are among the most sought-after, rare and expensive gemstones, along with diamonds, emeralds and the colour-changing alexandrites.

With a hardness of 9 on the Mohs scale, corundum is the second hardest mineral after diamond (excluding the very rare moissanite).



The special optical effect comes from the inclusion of rutile needles. If the stone has a silky lustre and appears cloudy to the eye, then a few rutile needles are aligned parallel to a crystal axis. The so-called chatoyance or cat's eye effect is caused by many rutile needles parallel to a crystal axis. When aligned parallel to the a-axes, the asterism or star effect occurs. Another special effect is the directional intergrowth of several ruby crystals, which together take on the appearance of a wheel with spokes. They are sold under the name "trapiche ruby".

It is believed that rubies were extracted and valued from pits in Burma as early as the Bronze Age. Over 2000 years ago, rubies were also worshipped in India and used as talismans. The ancient Egyptians, Greeks and Romans were also familiar with rubies.


Rubies are found on all continents except Antarctica. Asian rubies are the most sought after. Myanmar, Thailand and Sri Lanka, with its increasingly rare deposits, are the main exporters of these gemstones.

East African rubies (e.g. Kenya and Tanzania) are also highly prized. On the continents of North America (North Carolina/USA), South America (Colombia) and Australia there are only a few ruby occurrences.

In Europe, these gems have been found in Finland, Greenland, Norway and Macedonia. Until recently, Sri Lanka and India were the main producers of sapphires, but today the gems also come from the USA, particularly Montana, where sapphires are found in Yogo Gulch, Australia and Nigeria. Sapphires from Madagascar, more specifically Ilakaka, are considered to be of very high quality, but are usually declared as Sri Lankan, as they command higher prices. Production in Australia has declined sharply in recent years.

Crystallised trigonal
Hardness 9
Density 3.97 to 4.05 (ruby), 3.99 to 4.00 (sapphire)
Varieties: Ruby and Sapphire