Sapphire vs Spinel: How to Tell Them Apart
Blue sapphire and blue spinel can share a similar blue, but sapphire is one Mohs step harder and doubly refractive, while spinel is singly refractive and shows no pleochroism.
| Property | Sapphire | Spinel |
|---|---|---|
| Mineral | Corundum (Al2O3) | Magnesium aluminium oxide (MgAl2O4) |
| Typical colours | Blue (also pink, yellow, padparadscha and more) | Red, pink, blue, violet, orange, black |
| Mohs hardness | 9 | 8 |
| Toughness | Excellent | Excellent |
| Refractive index | 1.762–1.770 | 1.712–1.762 |
| Birefringence | 0.008 | None (singly refractive) |
| Optic character | Uniaxial – | Isotropic |
| Specific gravity | 3.95–4.03 | 3.54–3.63 |
| Pleochroism | Strong (blue / greenish blue in blue stones) | None (singly refractive) |
| Common treatments | Heat treatment is very common; diffusion and fracture filling also occur | Usually untreated; most gem spinel is sold as mined |
| Ultrasonic cleaning | Use caution | Generally safe |
How to tell Sapphire from Spinel
- DecisivePolariscope. Spinel is singly refractive and stays dark as you rotate it between crossed polarisers; Sapphire is doubly refractive and blinks light and dark four times per turn (single-crystal stones).
- DecisiveSpecific gravity. The SG ranges do not overlap (3.95–4.03 vs 3.54–3.63), so hydrostatic weighing or heavy liquids separate them.
- HelpfulRefractometer. The refractive index ranges only just touch (Sapphire 1.762–1.770 vs Spinel 1.712–1.762). A careful reading usually separates them, but take care near the boundary.
- HelpfulDichroscope. Pleochroism differs: Sapphire is Strong (blue / greenish blue in blue stones); Spinel is None (singly refractive).
- NoteDurability. Sapphire (Mohs 9) is harder than Spinel (Mohs 8). Do not scratch-test a stone you want to keep.
- NoteTreatments. Sapphire: Heat treatment is very common; diffusion and fracture filling also occur. Spinel: Usually untreated; most gem spinel is sold as mined.
Sapphire vs Spinel for jewellery
Sapphire has a Mohs hardness of 9 and excellent toughness; Spinel has a Mohs hardness of 8 and excellent toughness.
Both are durable enough for everyday jewellery. Spinel is usually untreated, while most sapphire is heat treated, so ask for disclosure.
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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 Spinel?
Blue sapphire and blue spinel can share a similar blue, but sapphire is one Mohs step harder and doubly refractive, while spinel is singly refractive and shows no pleochroism.
How can I tell Sapphire from Spinel?
Polariscope: Spinel is singly refractive and stays dark as you rotate it between crossed polarisers; Sapphire is doubly refractive and blinks light and dark four times per turn (single-crystal stones). Specific gravity: The SG ranges do not overlap (3.95–4.03 vs 3.54–3.63), so hydrostatic weighing or heavy liquids separate them.
Which is harder, Sapphire or Spinel?
Sapphire is harder: it is Mohs 9, while Spinel is Mohs 8.
Which is more durable for everyday jewellery, Sapphire or Spinel?
Sapphire has a Mohs hardness of 9 and excellent toughness; Spinel has a Mohs hardness of 8 and excellent 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 Spinel?
Sapphire: refractive index 1.762–1.770, specific gravity 3.95–4.03. Spinel: refractive index 1.712–1.762, specific gravity 3.54–3.63.
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