Corundum
| Colour | Red, Blue, Gray, Clear |
|---|---|
| Mohs hardness | 9 (defining mineral) |
| Lustre | Adamantine to vitreous |
| Streak | Colorless |
| Crystal system | Trigonal |
| Transparency | Transparent, translucent to opaque |
| Cleavage | None – parting in 3 directions |
| Fracture | Conchoidal to uneven |
| Chemical formula | Al 2 O 3 |
| Specific gravity | 3.95–4.10 |
What is Corundum?
Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium, and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange.
How to identify Corundum
- Lustre: Adamantine to vitreous.
- Hardness: Mohs 9 (defining mineral) — hard enough to scratch glass.
- Streak: Colorless.
- Habit: Trigonal crystal system.
Corundum in different forms
James St. John · CC BY 2.0 
Auckland Museum Collections from Auckland, Aotearoa New Zealand · CC BY 2.0 
Auckland Museum Collections from Auckland, Aotearoa New Zealand · CC BY 2.0 
Nessa Eull · CC0 
Darla Sondrol · CC0 
Nessa Eull · CC0 
James St. John · CC BY 2.0 
James St. John · CC BY 2.0 
James St. John · CC BY 2.0
Stones often confused with Corundum
Frequently asked questions
How hard is Corundum?
Corundum is Mohs 9 (defining mineral) on the hardness scale.
What colour is Corundum?
Corundum is typically red, blue, gray, clear (Colorless, gray, golden-brown, brown; purple, pink to red, orange, yellow, green, blue, violet; may be color zoned, aste).