Ruby vs Red Spinel: How to Tell Them Apart
Last updated: April 2026
Ruby (corundum) and red spinel are among the most commonly confused gem pairs in history. Many famous "rubies" in royal collections turned out to be spinel. The key difference: ruby is doubly refractive and spinel is singly refractive — a distinction that a polariscope or refractometer reveals in seconds.
Property Comparison
| Property | Ruby (Corundum) | Red Spinel | Diagnostic? |
|---|---|---|---|
| RI | 1.762 – 1.770 | 1.712 – 1.736 | Yes |
| SG | 3.97 – 4.05 | 3.57 – 3.63 | Yes |
| Hardness | 9 | 7.5 – 8 | No |
| Crystal System | Trigonal | Cubic | Yes |
| Optic Character | DR U- (doubly refractive) | SR (singly refractive) | Yes |
| Birefringence | 0.008 | None | Yes |
| Fluorescence (LW) | Variable (often strong red) | Variable (often strong red) | No |
| Chelsea Filter | Red | Variable | No |
| Pleochroism | Strong dichroic (purplish-red / orangey-red) | None | Yes |
The Definitive Tests
- Polariscope — optic character. Place the stone between crossed polars and rotate 360 degrees. Ruby blinks (alternating light and dark four times per rotation) because it is doubly refractive. Spinel stays dark throughout the rotation because it is singly refractive. This is the fastest and most definitive test.
- Refractometer — RI reading. Ruby reads 1.762-1.770 with a moving shadow edge (birefringence 0.008). Spinel reads 1.712-1.736 with a single non-moving edge. The ranges do not overlap — a single RI reading separates them.
- Dichroscope — pleochroism. Ruby shows strong dichroism: two distinct colors (purplish-red and orangey-red) visible simultaneously through the dichroscope. Spinel shows no pleochroism — both windows display the same color. This test works even on mounted stones.
Common Mistakes
- Relying on fluorescence alone. Both ruby and red spinel can show strong red fluorescence under LW UV. Fluorescence can support an identification but cannot separate these two species on its own.
- Assuming spinel always shows ADR. Spinel commonly exhibits anomalous double refraction (ADR) — a patchy or "tatami" extinction pattern on the polariscope that can be mistaken for true double refraction. Look for the consistent 4-position blink that indicates true DR.
- Forgetting historical context. Many antique and estate pieces labeled "ruby" are actually spinel. The Black Prince's Ruby in the British Crown Jewels is a well-documented example. Always test — do not rely on prior descriptions.
Frequently Asked Questions
Can you tell ruby from red spinel by color alone?
No. Fine red spinel and ruby can be virtually identical in color. Historically, many famous "rubies" in crown jewels were later identified as spinel. The separation requires measuring refractive index or testing optic character — not visual inspection.
What is the single best test to separate ruby from spinel?
The polariscope. Ruby is doubly refractive (DR, uniaxial negative) and blinks between light and dark as you rotate it between crossed polars. Spinel is singly refractive (SR, isotropic) and stays dark throughout the full rotation. This test takes seconds and is definitive.
Is red spinel less valuable than ruby?
Not necessarily. Fine red spinel commands strong prices — especially Burmese "jedi" spinel. However, equivalent quality ruby typically commands higher per-carat prices due to stronger market demand and name recognition.
Identifying a red stone? Loupewise walks you through polariscope and refractometer tests step by step.
Open Loupewise →Studying for a gemology practical? Loupewise’s timed exam simulators drill exactly this call — separating look-alike stones by their measured properties.
Practice exams →