Ruby vs Garnet: How to Tell Them Apart
Red garnets — particularly pyrope and almandine — are frequently mistaken for ruby because of overlapping color ranges. The separation is straightforward with a polariscope or refractometer: ruby is doubly refractive while all garnets are singly refractive, and RI/SG ranges differ significantly.
Property Comparison
| Property | Ruby (Corundum) | Pyrope Garnet | Almandine Garnet | Diagnostic? |
|---|---|---|---|---|
| RI | 1.762 – 1.770 | 1.730 – 1.760 | 1.770 – 1.830 | Yes |
| SG | 3.97 – 4.05 | 3.62 – 3.87 | 3.93 – 4.30 | Yes |
| Hardness | 9 | 7 – 7.5 | 7 – 7.5 | No |
| Crystal System | Trigonal | Cubic | Cubic | Yes |
| Optic Character | DR U- (doubly refractive) | SR (singly refractive) | SR (singly refractive) | Yes |
| Birefringence | 0.008 | None | None | Yes |
| Fluorescence (LW) | Variable (often strong red) | Inert | Inert | Yes |
| Chelsea Filter | Red | Variable | Inert | No |
| Pleochroism | Strong dichroic (purplish-red / orangey-red) | None | None | Yes |
The Definitive Tests
- Polariscope — optic character. Ruby blinks between light and dark during a 360-degree rotation (DR, uniaxial negative). Both pyrope and almandine garnet stay dark (SR, isotropic). Note: garnets may show anomalous double refraction (ADR) — a patchy, irregular extinction pattern — but this is distinguishable from ruby's clean 4-position blink.
- Dichroscope — pleochroism. Ruby shows strong dichroism: two distinct colors (purplish-red and orangey-red) visible simultaneously. All garnets show no pleochroism — both windows display the same color. This works even on mounted stones.
- Refractometer — RI reading. Ruby reads 1.762-1.770 with a moving shadow edge. Pyrope reads 1.730-1.760 with a single non-moving edge. Almandine reads 1.770-1.830 — note that almandine's RI overlaps with ruby's, but the single vs. double reading (no birefringence in garnet) separates them.
- LW UV fluorescence. Many rubies fluoresce strong red under LW UV. Pyrope and almandine garnets are typically inert. While this is not definitive on its own (some rubies are also inert, especially Thai/African origin), a strong red fluorescence supports ruby.
Common Mistakes
- Confusing almandine RI with ruby RI. Almandine garnet RI starts at 1.770 — overlapping ruby's range. Do not rely on RI alone. The key difference is that almandine shows a single, non-moving shadow edge (singly refractive), while ruby shows a moving edge with birefringence of 0.008.
- Ignoring anomalous double refraction (ADR) in garnet. Some garnets show ADR under the polariscope, which can be confused with true double refraction. ADR appears as a patchy, serpentine extinction — not the clean 4-position blink of corundum.
- Relying on color or hardness. Fine pyrope garnet can match ruby's red exactly. Hardness testing is destructive and impractical for finished stones. Property measurement — not appearance — is the path to identification.
Frequently Asked Questions
What is the easiest way to tell ruby from garnet?
Use a polariscope. Ruby is doubly refractive and blinks four times per rotation. Garnet is singly refractive and stays dark. This single test is definitive for separating corundum from garnet.
Can garnet be as red as ruby?
Yes. Fine pyrope garnet can show a rich, saturated red that closely resembles ruby. The distinction is not about color — it requires measuring physical and optical properties like RI, SG, and optic character.
Identifying a red stone? GemID walks you through polariscope and refractometer tests step by step.
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