Corundum Mineral | Meaning and Properties
Facts about Corundum
Corundum is a relatively common natural mineral that's a crystalline form of aluminium oxide (a chemical compound of aluminium and oxygen) with traces of iron, titanium and chromium. Naturally transparent it changes colour with the presence of impurities but in its purest form is colourless. Corundum is very hard and can scratch almost every other mineral on earth hence historically has been widely used as an abrasive although in more recent years it has gradually been replaced with a synthetic substitute.
On Mohs scale of mineral hardness corundum grades 9 so it's softer than diamond which is graded 10 but with that said, diamond is still four times harder.
Whilst few people are familiar with the mineral corundum they'll certainly be familiar with the gemstones ruby and sapphire both of which are gemstone varieties of corundum. With the presence of chromium the stone turns red but where it's lacking, impurities such as iron and titanium cause a variety of different colours including blue, yellow and black. Only red corundum is known as ruby whilst all other colours are known as sapphire.
Red Corundum - Photo Courtesy of Stan Celestian - Flickr
Inclusions of the mineral rutile in either red or blue corundum can result in an optical phenomenon known as asterism and when present, stones are better known as star ruby or star sapphire.
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Further ReadingInformation from Geology.com