Ruby vs Spinel: How to Tell Them Apart
Red spinel and ruby can look almost identical in colour, and many famous "rubies" in royal collections turned out to be spinel. The deciding difference is optical: ruby is doubly refractive while spinel is singly refractive.
| Property | Ruby | Spinel |
|---|---|---|
| Mineral | Corundum (Al2O3) | Magnesium aluminium oxide (MgAl2O4) |
| Typical colours | Pinkish red to deep red | 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 (purplish red / orangey red) | None (singly refractive) |
| Common treatments | Heat treatment is very common; lead-glass filling and diffusion also occur | Usually untreated; most gem spinel is sold as mined |
| Ultrasonic cleaning | Use caution | Generally safe |
How to tell Ruby from Spinel
- DecisivePolariscope. Spinel is singly refractive and stays dark as you rotate it between crossed polarisers; Ruby 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 (Ruby 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: Ruby is Strong (purplish red / orangey red); Spinel is None (singly refractive).
- NoteDurability. Ruby (Mohs 9) is harder than Spinel (Mohs 8). Do not scratch-test a stone you want to keep.
- NoteTreatments. Ruby: Heat treatment is very common; lead-glass filling and diffusion also occur. Spinel: Usually untreated; most gem spinel is sold as mined.
Ruby vs Spinel for jewellery
Ruby has a Mohs hardness of 9 and excellent toughness; Spinel has a Mohs hardness of 8 and excellent toughness.
Natural spinel is usually sold untreated and is often much less expensive than a ruby of similar colour and size, while rubies are commonly heat treated.
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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 Ruby and Spinel?
Red spinel and ruby can look almost identical in colour, and many famous "rubies" in royal collections turned out to be spinel. The deciding difference is optical: ruby is doubly refractive while spinel is singly refractive.
How can I tell Ruby from Spinel?
Polariscope: Spinel is singly refractive and stays dark as you rotate it between crossed polarisers; Ruby 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, Ruby or Spinel?
Ruby is harder: it is Mohs 9, while Spinel is Mohs 8.
Which is more durable for everyday jewellery, Ruby or Spinel?
Ruby 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 Ruby and Spinel?
Ruby: 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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