Alexandrite vs Color-Change Garnet: How to Tell Them Apart
Both alexandrite (chrysoberyl) and color-change garnet (pyrope-spessartite) exhibit dramatic color change — green to red under different light sources. Their RI ranges overlap (1.746-1.755 vs 1.740-1.765), making RI alone unreliable. The key separation is optic character: alexandrite is doubly refractive and garnet is singly refractive. The polariscope resolves the question in seconds.
Property Comparison
| Property | Alexandrite (Chrysoberyl) | Color-Change Garnet | Diagnostic? |
|---|---|---|---|
| RI | 1.746 – 1.755 | 1.740 – 1.765 | No |
| SG | 3.70 – 3.78 | 3.65 – 3.80 | No |
| Hardness | 8.5 | 7 – 7.5 | No |
| Crystal System | Orthorhombic | Cubic | Yes |
| Optic Character | DR B+ (doubly refractive, biaxial) | SR (singly refractive, isotropic) | Yes |
| Birefringence | 0.009 | None | Yes |
| Fluorescence (LW) | Moderate red | Inert | Yes |
| Chelsea Filter | Red | Variable | No |
| Pleochroism | Strong trichroic (green / orange / purple-red) | None | Yes |
The Definitive Tests
- Polariscope — optic character (definitive). Place the stone between crossed polars and rotate 360 degrees. Alexandrite blinks distinctly (doubly refractive, biaxial positive). Color-change garnet stays dark throughout the rotation (singly refractive, isotropic). This single test separates them definitively, even though their RI and SG ranges overlap.
- Dichroscope — pleochroism. Alexandrite shows strong trichroism: three distinct colors visible as you rotate through different orientations (green, orange, and purple-red). Color-change garnet shows no pleochroism — both windows display the same color regardless of orientation. This is the second fastest test and works on mounted stones.
- UV fluorescence (LW). Alexandrite typically shows moderate red fluorescence under LW UV. Color-change garnet is typically inert. A moderate red LW fluorescence in a color-change stone supports alexandrite identification.
Common Mistakes
- Relying on RI to separate them. Alexandrite RI 1.746-1.755 and color-change garnet RI 1.740-1.765 overlap significantly. An RI reading of 1.750 is consistent with either species. The single vs. double refraction (observed via polariscope or refractometer shadow-edge behavior) is what matters — not the absolute RI value.
- Confusing color-change quality with species. Color-change garnet sometimes shows a blue-green to red shift that closely mimics alexandrite. The quality or character of the color change does not identify the species. Only physical property testing can distinguish them.
- Forgetting synthetic alexandrite. Synthetic alexandrite (grown by Czochralski or flux methods) is also doubly refractive and shows trichroism — it will test the same as natural alexandrite on portable instruments. Separating natural from synthetic alexandrite requires microscopic inclusion analysis, which is a different question from the species identification addressed here.
Frequently Asked Questions
Can RI separate alexandrite from color-change garnet?
Not reliably. Alexandrite RI is 1.746-1.755 and color-change garnet RI is 1.740-1.765 — the ranges overlap. The polariscope is the definitive separator: alexandrite blinks (doubly refractive, biaxial positive) while garnet stays dark (singly refractive, isotropic).
Which is more valuable — alexandrite or color-change garnet?
Natural alexandrite is one of the rarest and most valuable gemstones, with fine examples commanding tens of thousands of dollars per carat. Color-change garnet is considerably less expensive, though fine specimens are still valued by collectors. The identification matters enormously for valuation.
What is the fastest way to tell them apart?
The polariscope. Place the stone between crossed polars and rotate 360 degrees. Alexandrite blinks (light-dark-light-dark) because it is biaxial. Color-change garnet stays dark because it is isotropic. This takes seconds and is the most definitive field test.
Identifying a color-change stone? GemID walks you through polariscope and dichroscope tests step by step.
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