What is corundum?
By Jared James · Last updated 21 May 2026
Corundum is the mineral species that includes both ruby and sapphire. It is aluminium oxide in crystalline form, one of the hardest natural minerals at 9 on the Mohs scale, which is why both stones are so durable and so widely used in fine jewellery. When trace amounts of chromium colour the crystal red, the stone is a ruby; blue corundum coloured by iron and titanium is sapphire; and corundum in any other colour, from pink through yellow to green, is also called sapphire, qualified by its colour.
Frequently asked questions
- Is a ruby the same as a red sapphire?
- In mineral terms they are both corundum, but in the trade they are treated as separate gemstones. The dividing line is the depth of the red, and it is a subject of ongoing debate among gemmologists, particularly for stones in the pinkish-red range.
- Why is corundum so hard?
- Corundum's hardness comes from the strong bonding between aluminium and oxygen in its crystal structure. At 9 on the Mohs scale it is second only to diamond, which makes it highly resistant to scratching and ideal for rings and other pieces that take daily wear.
- What is synthetic corundum?
- Synthetic corundum is genuine corundum grown in a laboratory rather than mined, and it has the same chemical composition, hardness and optical properties as natural ruby or sapphire. It is used in affordable jewellery and also in watchmaking and industrial applications.
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