Muséum national d'histoire naturelle

GENERAL DESCRIPTION

Corundum is an aluminium oxide (or "alumina" for the industrials). It is one of the most known minerals, thanks to the gems "rubies" and "sapphire" which constitute its red varieties and blue, respectively. It is formed mainly during the metamorphism of high temperature affecting rocks enriched in aluminium but low in silica.

The crystals are more or less flattened hexagonal prisms and sometimes bipyramidal. They are transparent and of varied colours.

The varieties of corundum are defined according to their colour: the ruby indicates corundum red (but not pink or purple) and sapphire indicates corundums of the other colours (often blue but also yellow, purple...). The very rare orange corundum is named "Padparadscha sapphire" and the ruby with a major red clor (slightly bluish) is named " pigeon’s blood".

Colours are due to chromium impurities (ruby), titanium and iron (blue sapphire), vanadium (purple sapphire). Rutile inclusions in some crystals cause an asterism with 6 branches called "star corundum".

A luminous star is visible at the surface of the polished cabochons. The stones of gem quality are rare and exceed only seldomly a few centimetres. Ruby and sapphire, just like diamond and the emerald, are qualified true precious stones. But the ruby is, by far, the rarest of the four.

The corundum deposits are rather numerous, but the mines producing gem-quality pieces are rarer, like those of the district of Mogok in Burma which produces the most beautiful rubies. In Sri Lanka, sapphire is associated many other gems : it has been mined for more than 25 centuries (sapphires of Ceylon, old name of "Sri Lanka"). It was found opaque crystals in the district of Letaba in South Africa, weighing a little more than one hundred of kilos.

In France, sapphire was found at the Riou Pezzouliou, close of Espaly-Saint-Marcel in Haute-Loire (and in the volcanic chimneys of the surroundings) and more recently in Puy de Dôme. More generally, the corundum was reported in many other deposits of the Massif Central (the Limousin, Cantal etc) like in Brittany (Finistère, Morbihan), close to Nantes (Loire-Atlantique), in the Rhône (Chessy).

The type is not definable because this species was already known before the conditions of deposit of the types were defined.

Did you know? The multitude of possible colours and their high hardness make them appreciated stones in jewellery. Known since Antiquity under other names, rubies and sapphires ornamente the most prestigious jewels. The first synthetic corundums were manufactured about 1890 by Verneuil and Frémy thanks to their "process of fusion by flames".

Corundum

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Mineralogy