Pyrope Garnet vs Spinel: How to Tell Them Apart
Last updated: August 2026
Pyrope garnet and red spinel are the classic singly-refractive red pair — and one of the hardest calls a standard instrument set has to make. Both are SR, both are cubic, and their RI ranges overlap between 1.730 and 1.736. This is exactly the kind of separation gemology practical exams are built around: when the refractometer alone cannot decide, the spectroscope, UV lamp, and microscope have to.
Property Comparison
| Property | Pyrope Garnet | Spinel | Diagnostic? |
|---|---|---|---|
| RI | 1.730 – 1.760 | 1.712 – 1.736 | No |
| SG | 3.62 – 3.87 | 3.57 – 3.63 | No |
| Hardness | 7 – 7.5 | 7.5 – 8 | No |
| Crystal System | Cubic | Cubic | No |
| Optic Character | SR (singly refractive) | SR (singly refractive) | No |
| Birefringence | None | None | No |
| Fluorescence (LW) | Inert | Variable (strong in red/pink varieties) | Yes |
| Chelsea Filter | Variable | Variable | No |
| Pleochroism | None | None | No |
| Spectroscope | Almandine (iron) spectrum | No almandine spectrum | Yes |
The Definitive Tests
- Spectroscope — the almandine spectrum. Pyrope garnet shows the iron absorption spectrum inherited from its almandine component; spinel does not. When the RI reading lands in the 1.730-1.736 overlap zone, this is the test that decides the call.
- LW UV — fluorescence. Pyrope is inert under long-wave UV. Spinel's fluorescence is variable, but red and pink spinels commonly fluoresce strongly. A distinct red glow under the lamp points away from garnet.
- Refractometer — RI position. Spinel reads 1.712-1.736; pyrope reads 1.730-1.760. A reading below 1.730 indicates spinel and a reading above 1.736 indicates pyrope. Only the narrow 1.730-1.736 overlap is ambiguous — that is when you reach for the spectroscope. Under the microscope, spinel's characteristic octahedral crystals are a further pointer; per the reference data, inclusions help separate the pair.
Common Mistakes
- Stopping at “SR and red.” Both stones stay dark under crossed polars and both are red. Singly refractive plus red body color is where this identification starts, not where it ends — candidates who write down “garnet” at that point are guessing.
- Treating SG as a separator. Pyrope's SG range (3.62-3.87) overlaps spinel's (3.57-3.63) at the bottom end. A hydrostatic reading near 3.6 is consistent with either stone; only a clearly higher value favors pyrope.
- Relying on the Chelsea filter. Both pyrope and spinel give variable Chelsea filter reactions. The filter cannot separate this pair — use the spectroscope and UV lamp instead.
Frequently Asked Questions
Can the refractometer separate pyrope garnet from spinel?
Sometimes. Spinel reads 1.712-1.736 and pyrope reads 1.730-1.760, so readings outside the 1.730-1.736 overlap are decisive. Inside the overlap, the refractometer alone cannot make the call — the spectroscope and UV fluorescence separate them.
What is the best test to separate pyrope from spinel?
The spectroscope. Pyrope shows an almandine (iron) absorption spectrum that spinel lacks. Long-wave UV supports it: pyrope is inert, while red and pink spinels commonly fluoresce strongly.
Why do exams love the pyrope-vs-spinel separation?
Because both stones are singly refractive, cubic, and red, with overlapping RI and SG. It is one of the few common separations where the refractometer and polariscope can both fail to decide, forcing candidates to use the spectroscope, UV lamp, and microscope correctly.
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