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AuAL said:
Sapphires and rubies are the same mineral, corundum (Al2O3, i.e. aluminium “rust”.) They were named before mineral composition was understood, their names referencing their (predominant) colours. The name sapphire now covers corundum of any colour of the rainbow except red, which is a ruby. Rubies are only of a restricted range of red. Deviate too far and you have a sapphire (and the value goes down.)

(Because corundum has a crystal structure with very strong bonds, second only to diamond in hardness, and pure aluminium is very reactive, the surface of aluminium forms a strong protective layer of oxide very rapidly, preventing oxygen from reaching the aluminium below. So aluminium will not “rust away” like iron.)
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Thank you!
 
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