Corundum is common as an accessory mineral in metamorphic rocks and as an original constituent of certain igneous rocks. Pure corundum is colourless, but colour differences due to the presence of other elements give rise to several varieties of gem, notably, ruby and saphire. The deep red ruby (corundum tainted with chromium) is one of the most valuable gems, second only to emerald and diamond. Sapphire is corundum tainted with titanium. Corundum has the formulae Al2O3 and a relative hardness of 9.
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