Sapphire vs Tanzanite: How to Tell Them Apart
Both stones can be violet-blue, but sapphire is far harder and tougher, while tanzanite is softer, shows strong three-colour pleochroism and is almost always heated.
| Property | Sapphire | Tanzanite |
|---|---|---|
| Mineral | Corundum (Al2O3) | Zoisite (Ca2Al3Si3O12(OH)) |
| Typical colours | Blue (also pink, yellow, padparadscha and more) | Violet-blue to blue-violet |
| Mohs hardness | 9 | 6–7 |
| Toughness | Excellent | Poor |
| Refractive index | 1.762–1.770 | 1.691–1.700 |
| Birefringence | 0.008 | 0.010 |
| Optic character | Uniaxial – | Biaxial + |
| Specific gravity | 3.95–4.03 | 3.10–3.38 |
| Pleochroism | Strong (blue / greenish blue in blue stones) | Strong, three colours (blue / violet / burgundy) |
| Common treatments | Heat treatment is very common; diffusion and fracture filling also occur | Heat treatment is standard; nearly all tanzanite is heated |
| Ultrasonic cleaning | Use caution | Avoid |
How to tell Sapphire from Tanzanite
- DecisiveRefractometer. The refractive index ranges do not overlap (Sapphire 1.762–1.770 vs Tanzanite 1.691–1.700), so a single reading separates them.
- DecisiveSpecific gravity. The SG ranges do not overlap (3.95–4.03 vs 3.10–3.38), so hydrostatic weighing or heavy liquids separate them.
- HelpfulOptic character. Both are doubly refractive, but Sapphire is Uniaxial – and Tanzanite is Biaxial +. A conoscope or an optic-sign test on a refractometer can tell them apart.
- NoteDurability. Sapphire (Mohs 9) is harder than Tanzanite (Mohs 6–7). Do not scratch-test a stone you want to keep.
- NoteTreatments. Sapphire: Heat treatment is very common; diffusion and fracture filling also occur. Tanzanite: Heat treatment is standard; nearly all tanzanite is heated.
Sapphire vs Tanzanite for jewellery
Sapphire has a Mohs hardness of 9 and excellent toughness; Tanzanite has a Mohs hardness of 6–7 and poor toughness.
Sapphire suits rings worn daily. Tanzanite is better in earrings and pendants, or in rings worn with care.
Educational reference only. Typical published values vary between specimens and sources, and this page is not a gemological laboratory report. Confirm important stones with a recognised gem lab.
Frequently asked questions
What is the main difference between Sapphire and Tanzanite?
Both stones can be violet-blue, but sapphire is far harder and tougher, while tanzanite is softer, shows strong three-colour pleochroism and is almost always heated.
How can I tell Sapphire from Tanzanite?
Refractometer: The refractive index ranges do not overlap (Sapphire 1.762–1.770 vs Tanzanite 1.691–1.700), so a single reading separates them. Specific gravity: The SG ranges do not overlap (3.95–4.03 vs 3.10–3.38), so hydrostatic weighing or heavy liquids separate them.
Which is harder, Sapphire or Tanzanite?
Sapphire is harder: it is Mohs 9, while Tanzanite is Mohs 6–7.
Which is more durable for everyday jewellery, Sapphire or Tanzanite?
Sapphire has a Mohs hardness of 9 and excellent toughness; Tanzanite has a Mohs hardness of 6–7 and poor toughness. Toughness describes resistance to chipping and breaking, which matters as much as hardness for rings.
What are the refractive index and specific gravity of Sapphire and Tanzanite?
Sapphire: refractive index 1.762–1.770, specific gravity 3.95–4.03. Tanzanite: refractive index 1.691–1.700, specific gravity 3.10–3.38.
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