Peridot vs Sinhalite: How to Tell Them Apart
Last updated: August 2026
Sinhalite was originally misidentified as a brown variety of peridot and only described as its own gem species in 1952 — and it remains a beloved trap in gemology practicals, a Gem-A FGA classic. The RI ranges overlap (1.665-1.690), both stones are biaxial with strong visible doubling, and the SG ranges touch. The separation rests on the spectroscope: sinhalite's three-band absorption pattern at 452, 475, and 493 nm is essentially unique to it.
Property Comparison
| Property | Peridot (Olivine) | Sinhalite | Diagnostic? |
|---|---|---|---|
| RI | 1.654 – 1.690 | 1.665 – 1.712 | No |
| SG | 3.27 – 3.48 | 3.47 – 3.50 | No |
| Hardness | 6.5 – 7 | 6.5 – 7 | No |
| Crystal System | Orthorhombic | Orthorhombic | No |
| Optic Character | DR B+ (biaxial positive) | DR B− (biaxial negative) | No |
| Birefringence | 0.036 | 0.038 | No |
| Facet Doubling | Extreme | Strong | No |
| Fluorescence (LW) | Inert | Inert | No |
| Pleochroism | Weak (yellow-green / green / colorless) | Strong trichroic (pale brown / dark brown / greenish brown) | Yes |
| Spectroscope | No 452/475/493 nm band set | Absorption bands at 452, 475, and 493 nm | Yes |
The Definitive Tests
- Spectroscope — the three-band pattern. Sinhalite shows diagnostic absorption bands at 452, 475, and 493 nm — a pattern essentially unique to sinhalite. This is the definitive call, and it is exactly the kind of spectrum reading the FGA practical rewards.
- Refractometer — the upper shadow edge. The ranges overlap between 1.665 and 1.690, but only sinhalite reads above 1.690 (up to 1.712). An upper reading beyond 1.690 rules peridot out. A reading inside the overlap zone means the refractometer alone cannot decide — go to the spectroscope.
- SG and pleochroism — supporting evidence. Per the reference data, SG separates at the upper end: sinhalite sits at 3.47-3.50 while peridot spans 3.27-3.48 with a typical value of 3.34, so most peridot weighs in clearly lighter. The dichroscope adds a pointer: sinhalite is strongly trichroic in browns (pale brown / dark brown / greenish brown), while peridot's pleochroism is weak.
Common Mistakes
- Calling every doubled yellow-green stone peridot. Both stones show conspicuous doubling under the loupe — peridot's is extreme (birefringence 0.036) and sinhalite's is strong (0.038). Doubling plus olive color is consistent with both candidates; it cannot close the identification.
- Deciding on an RI reading inside the overlap. A shadow edge between 1.665 and 1.690 is consistent with either stone. Only readings above 1.690 (sinhalite) or below 1.665 (peridot) are decisive on their own. Record the full reading, then confirm with the spectroscope.
- Skipping the spectroscope on brownish material. Brownish or brownish-green “peridot” is precisely where sinhalite hides — that is how it was originally misidentified. The 452/475/493 nm band set takes seconds to check and settles the question.
Frequently Asked Questions
How was sinhalite discovered hiding as peridot?
Sinhalite was originally misidentified as a brown variety of peridot: the two overlap in RI, show similar strong doubling, and occur in similar colors. It was first described as a distinct gem species in 1952, and the separation has been a gemology-exam favorite ever since.
What is the definitive test to separate peridot from sinhalite?
The spectroscope. Sinhalite shows absorption bands at 452, 475, and 493 nm — a pattern essentially unique to it. Supporting evidence: sinhalite's RI can read up to 1.712 (above peridot's 1.690 maximum) and its SG of 3.47-3.50 sits at the very top of peridot's range.
Can specific gravity separate peridot from sinhalite?
Usually, but not always. Sinhalite is 3.47-3.50 while peridot spans 3.27-3.48 (typically 3.34), so the ranges technically touch at the top. A low reading indicates peridot; a reading of 3.47+ favors sinhalite but should be confirmed with the spectroscope.
Identifying a yellow-green or brownish stone? GemID walks you through refractometer and spectroscope tests step by step.
Open GemID →Preparing for the FGA practical? Peridot vs sinhalite is a Gem-A syllabus classic. GemID’s timed exam simulators drill exactly this call — separating look-alike stones by their measured properties.
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