Corundum
“Rubies and sapphires in one mineral”
Corundum is an aluminum oxide mineral and the second-hardest natural substance after diamond. In its pure form it's colorless, but trace chromium creates red ruby while iron and titanium produce blue sapphire — two of the most valuable gemstones on Earth, both from the same mineral.
aluminum oxide (Al₂O₃)
Myanmar (ruby), Kashmir and Sri Lanka (sapphire), Montana (USA), Thailand, Madagascar, Tanzania
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Synthetic corundum is grown industrially in massive quantities — it's used in everything from watch faces to spacecraft windows, and it's the material in the scratch-resistant glass on many smartphones.
Test Your Knowledge
What trace element creates the blue colour in sapphire and the red in ruby — both varieties of corundum?
Photo Attribution
Related Specimens
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