What is Corundum?
Corundum is an exceptionally hard aluminium oxide and the mineral species that includes the gemstone varieties ruby and sapphire.
Mineralogy and structure
The reference formula is Al2O3. Corundum belongs to the oxides and has a trigonal crystal system. Corundum has a trigonal structure and Mohs hardness 9, second only to diamond among common natural minerals. Colour is controlled by trace elements such as chromium, iron, titanium and vanadium.
Colour and appearance
Typical colours include colourless, grey, blue, red, pink, yellow, green. Crystal habit, lustre, transparency and matrix association are often as diagnostic as colour.
Formation and localities
Corundum forms in or is associated with aluminium-rich metamorphic rocks, marble, basaltic environments and syenitic or pegmatitic rocks. Important documented localities include Myanmar, Sri Lanka, Madagascar, India, Tanzania, Thailand and Montana in the United States.
Collecting and care
Corundum is best kept as a labelled collector or reference specimen. Protect fragile crystals from impact and prolonged humidity changes. Lead-, arsenic-, chromium- and cobalt-bearing minerals should not be cut, ground or used in jewellery.
