Corundum Mineral | Meaning and Properties

 

 

rough blue corundum mineral on white background

 

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 the stone is colourless.  Corundum is one of the hardest minerals known to man 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 and it's four times as hard as corundum. 

Whilst people may be less familiar with the name of this mineral they'll certainly know about rubies and sapphires both of which are gemstone varieties of corundum.  With the presence of chromium corundum turns red but where chromium is lacking impurities such as iron and titanium cause a variety of different colours including blue, yellow and black.  Only when red is corundum known as ruby whilst all other colours are known as sapphire.

 

 

large piece of the mineral red corundum

Red Corundum - Photo; Stan Celestian. Click to Enlarge

 

 

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 Reading

Information from Geology.com