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The word corundum originates from the Sanskrit word kurivinda. Corundum is a mineral to which both rubies and sapphires belong. The chemical composition of corundum is aluminium oxide (Al2 O3) with traces of various transition elements such as chromium (Cr). It is these transition elements that are responsible for producing the stone's colour. Chromium produces the finest red colour in rubies and also pink coloured sapphires. Chromium produces some of the finest colours found in many other gems like emeralds, jadeite, and chrome diopside to name a few. Other transition elements found in corundum include iron (Fe), vanadium (V) and titanium (Ti). Gem quality corundum comes in all colours with red corundum described as ruby and all other colours described as a sapphire with the name of the colour as a prefix. Corundum crystallizes in a hexagonal (trigonal) form with the six crystal faces meeting at 120 degrees. |
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Physical Properties of Corundum
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Refractive Index |
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1.762 - 1.770 |
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Birefringence |
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0.008 - 0.009 |
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Crystal System |
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Hexagonal (trigonal) |
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Optic Character |
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Uniaxial (-) |
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Specific Gravity |
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3.97 - 4.03 |
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Hardness |
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9 |
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Pleochroism |
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Very Pronounced |
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Chemical Composition |
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Aluminium Oxide (Al2 O3) |
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Cleavage |
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None |
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Dispersion |
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0.008 |
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The cutting stages of a 13 carat pink sapphire |
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