Diamond vs Moissanite: How to Tell Them Apart
Last updated: April 2026
Moissanite is the one diamond simulant that fools thermal testers. Both read as "diamond" on standard thermal probes because moissanite's thermal conductivity is close to diamond's. The separation requires a different approach: birefringence, facet doubling, and electrical conductivity testing.
Property Comparison
| Property | Diamond | Moissanite | Diagnostic? |
|---|---|---|---|
| RI | 2.417 | 2.648 – 2.691 | No |
| SG | 3.50 – 3.53 | 3.20 – 3.22 | No |
| Hardness | 10 | 9.25 | No |
| Crystal System | Cubic | Hexagonal | Yes |
| Optic Character | SR (singly refractive) | DR U+ (doubly refractive) | Yes |
| Birefringence | None | 0.043 (extreme) | Yes |
| Dispersion | 0.044 | 0.104 | Yes |
| Facet Doubling | None | Extreme — visible under 10x | Yes |
| Fluorescence (LW) | Variable (often blue) | Variable (often orange/yellow) | No |
| Chelsea Filter | Inert | Inert | No |
| Pleochroism | None | Weak | No |
| Thermal Test | Reads "diamond" | Also reads "diamond" | No |
The Definitive Tests
- Facet doubling under 10x loupe. Look through the table facet at the back facet edges. In moissanite, every edge appears doubled due to extreme birefringence (0.043). Diamond is singly refractive and shows single, crisp edges. This is the fastest field test.
- Dedicated moissanite tester (electrical conductivity). Moissanite is an electrical semiconductor; diamond (except rare Type IIb blue) is an insulator. A dual-mode tester that checks both thermal and electrical conductivity will correctly flag moissanite. Standard thermal-only testers cannot separate them.
- Dispersion comparison. Moissanite's dispersion (0.104) is more than double diamond's (0.044). Under bright point-source lighting, moissanite shows noticeably more spectral fire — sometimes described as "rainbow flash." This is subjective but experienced graders notice it immediately.
- SG measurement. If the stone is loose, hydrostatic SG measurement separates them: diamond reads 3.50-3.53, moissanite reads 3.20-3.22. Moissanite is measurably lighter for the same volume.
Common Mistakes
- Trusting a thermal tester alone. This is the most common error. Standard thermal testers report moissanite as "diamond" because both materials have high thermal conductivity. Always confirm with a moissanite (electrical) tester or the facet-doubling test.
- Missing facet doubling in small stones. In stones under 0.5 ct, the doubling effect is subtle and easy to overlook. Tilt the stone and look through the crown at an angle to maximize the optical path through the birefringent material.
- Assuming RI separates them on a refractometer. Both diamond (RI 2.417) and moissanite (RI 2.648-2.691) read above the limit of a standard gemological refractometer (~1.81). Neither gives a readable shadow edge, so RI cannot be used for separation with standard equipment.
Frequently Asked Questions
Can a diamond tester distinguish diamond from moissanite?
A standard thermal diamond tester cannot — moissanite reads as "diamond" because both have high thermal conductivity. You need a dedicated moissanite tester (electrical conductivity) or a dual-mode tester that checks both thermal and electrical properties.
What is the fastest way to tell diamond from moissanite?
Look for facet doubling under 10x magnification. Moissanite has extreme birefringence (0.043), so the back facet edges appear doubled when viewed through the table. Diamond is singly refractive and never shows facet doubling.
Is moissanite natural or always synthetic?
All gem-quality moissanite on the market is synthetic (lab-created silicon carbide). Natural moissanite exists only as microscopic grains in meteorites and certain geological formations — it has never been found in gem quality.
Identifying a diamond or moissanite? Loupewise walks you through these 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.
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