RocksdayCorundum vs Kyanite
How to tell them apart
Aluminum oxide and the second-hardest natural mineral. Red corundum is ruby; every other color is sapphire.
Photo: Ra'ike (see also Ra'ike on de.wikipedia, CC BY-SA 3.0
A blue, bladed metamorphic mineral famous for having different hardness in different directions.
Photo: User:Aelwyn, CC BY-SA 3.0
Quick tests
- Hardness: Corundum is clearly harder: Mohs 9 versus 4.5–7.
Key differences
- Kyanite is blue in flat blades and can be scratched along its length.
- Sapphire is hardness 9 in every direction and forms barrels, not blades.
Side by side
| Property | Corundum | Kyanite |
|---|---|---|
| Type | Mineral | Mineral |
| Mohs hardness | 9 | 4.5–7 |
| Luster | Adamantine to vitreous | Vitreous to pearly |
| Streak | White | White |
| Colors | Red, Blue, Pink, Gray | Blue, Gray, Green |
| Crystal system | Trigonal | Triclinic |
| Specific gravity | 3.95–4.1 | 3.53–3.67 |
| Formula | Al₂O₃ | Al₂SiO₅ |
| Found in | Mogok, Myanmar; Sri Lanka | Minas Gerais, Brazil; Nepal |
Properties that are the same for both are grayed out.
How to identify corundum
- Hardness 9 — scratches everything except diamond.
- Barrel-shaped six-sided crystals.
- Heavy for a non-metallic stone.
- Often shows flat parting planes.
Crystal system: Trigonal
Full Corundum guide →How to identify kyanite
- Long, flat, blue blades with streaky color.
- A knife scratches it along the length but not across.
- Splits along the blades.
- Found in schist and gneiss.
Crystal system: Triclinic
Full Kyanite guide →More comparisons
Could you tell them apart from a blurry close-up?
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