Gemstone Comparator
Compare any two gemstones side by side: hardness, refractive index, specific gravity, pleochroism, treatments, and the exact tests that tell look-alikes apart.
| 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 them apart
- 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.
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.
Popular gemstone comparisons
Each comparison below has its own page with a full table and testing guide.
- Ruby vs Spinel
- Sapphire vs Spinel
- Sapphire vs Tanzanite
- Emerald vs Peridot
- Ruby vs Garnet (pyrope / almandine)
- Aquamarine vs Topaz
How to compare gemstones
No single property identifies a gemstone, so gemologists combine several. The comparator lines up the properties that matter most and tells you which are decisive for your pair.
- Refractive index (RI) measured on a refractometer. If two RI ranges do not overlap, one reading is enough.
- Optic character checked in a polariscope. Singly refractive stones stay dark; doubly refractive stones blink.
- Specific gravity (SG) found by weighing the stone in air and in water. Use our specific gravity calculator.
- Pleochroism seen through a dichroscope. Strongly pleochroic stones show different colours from different directions.
- Inclusions and treatments studied under a loupe or microscope.
If you already have a refractive index reading, try the Gemstone Identifier to see which species it could be, then compare the candidates here.
Gemstone reference table
| Gemstone | Refractive index | Specific gravity | Mohs | Optic character |
|---|---|---|---|---|
| Ruby | 1.762–1.770 | 3.95–4.03 | 9 | Uniaxial – |
| Sapphire | 1.762–1.770 | 3.95–4.03 | 9 | Uniaxial – |
| Spinel | 1.712–1.762 | 3.54–3.63 | 8 | Isotropic |
| Emerald | 1.577–1.583 | 2.67–2.80 | 7.5–8 | Uniaxial – |
| Aquamarine | 1.577–1.583 | 2.67–2.80 | 7.5–8 | Uniaxial – |
| Tanzanite | 1.691–1.700 | 3.10–3.38 | 6–7 | Biaxial + |
| Peridot | 1.654–1.690 | 3.28–3.48 | 6.5–7 | Biaxial + |
| Garnet (pyrope / almandine) | 1.730–1.810 | 3.65–4.30 | 6.5–7.5 | Isotropic |
| Topaz | 1.609–1.637 | 3.49–3.57 | 8 | Biaxial + |
| Tourmaline | 1.614–1.666 | 2.82–3.32 | 7–7.5 | Uniaxial – |
| Zircon (high type) | 1.920–1.985 | 3.90–4.73 | 6–7.5 | Uniaxial + |
| Diamond | 2.417 | 3.50–3.53 | 10 | Isotropic |
| Chrysoberyl / Alexandrite | 1.746–1.755 | 3.68–3.78 | 8.5 | Biaxial + |
| Quartz (amethyst / citrine) | 1.544–1.553 | 2.63–2.65 | 7 | Uniaxial + |
| Kunzite | 1.660–1.676 | 3.15–3.20 | 6.5–7 | Biaxial + |
| Iolite | 1.542–1.551 | 2.57–2.66 | 7–7.5 | Biaxial – |
Frequently asked questions
How do I compare two gemstones?
Choose a gemstone in each box and the comparator shows hardness, toughness, refractive index, birefringence, optic character, specific gravity, pleochroism, common treatments and cleaning safety side by side, with a list of the tests that separate them.
Which tests separate look-alike gemstones?
A refractometer, a polariscope and specific gravity are the three standard tests. A refractometer separates stones whose refractive index ranges do not overlap, a polariscope separates singly refractive from doubly refractive stones, and specific gravity separates stones of different density.
What is the difference between singly and doubly refractive?
Singly refractive (isotropic) stones such as spinel, garnet and diamond bend light in one direction and stay dark between crossed polarisers. Doubly refractive stones such as ruby, sapphire and emerald split light in two and blink light and dark as you rotate them.
Why do the ranges overlap for some gemstones?
Each gemstone species has a range of values because chemistry and impurities vary between specimens. When two ranges overlap, one measurement cannot separate them and you need a second property or a lab report.
Are the values on this page exact?
They are typical published ranges, not measurements of your stone. Your own readings can fall slightly outside a range, and rare varieties can differ. Treat the result as a guide and confirm valuable stones with a gemological laboratory.
Can I identify a gemstone from a photo?
No. Colour alone cannot identify a gemstone because many species share colours. Use measured properties such as refractive index and specific gravity, then compare them here.
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