Diamond vs Cubic Zirconia: How to Tell Them Apart
Cubic zirconia (CZ) is the most common diamond simulant in the market. Unlike moissanite, CZ is easily separated from diamond by a standard thermal tester. CZ is also dramatically heavier — its SG of 5.6-6.0 makes it about 65% denser than diamond, a difference you can feel when comparing same-size stones.
Property Comparison
| Property | Diamond | Cubic Zirconia | Diagnostic? |
|---|---|---|---|
| RI | 2.417 | 2.150 – 2.180 | No |
| SG | 3.50 – 3.53 | 5.60 – 6.00 | Yes |
| Hardness | 10 | 8 – 8.5 | No |
| Crystal System | Cubic | Cubic | No |
| Optic Character | SR (singly refractive) | SR (singly refractive) | No |
| Birefringence | None | None | No |
| Dispersion | 0.044 | 0.066 | Yes |
| Fluorescence (LW) | Variable (often blue) | Inert | No |
| Chelsea Filter | Inert | Inert | No |
| Pleochroism | None | None | No |
| Thermal Test | Reads "diamond" | Reads "simulant" | Yes |
The Definitive Tests
- Thermal diamond tester. CZ reads "simulant" on any standard thermal tester because it has much lower thermal conductivity than diamond. This is the single fastest test and works on mounted stones. (Note: moissanite also reads "diamond" on thermal testers — CZ does not.)
- SG measurement (loose stones). CZ weighs dramatically more than diamond for the same size: SG 5.60-6.00 vs diamond's 3.50-3.53. A one-carat-sized CZ weighs roughly 1.65 carats. This difference is also noticeable by hand — CZ feels heavier than expected.
- Dispersion observation. Under point-source lighting, CZ shows more fire (rainbow flashes) than diamond because its dispersion (0.066) is 50% higher. Trained observers notice this immediately, though it requires experience.
- Loupe inspection — facet edges and wear. CZ (hardness 8-8.5) wears faster than diamond (hardness 10). Used CZ typically shows rounded facet edges and surface abrasion under 10x, while diamond maintains sharp edges.
Common Mistakes
- Attempting to use a standard refractometer. Both diamond (RI 2.417) and CZ (RI 2.15-2.18) read above the limit of standard gemological refractometers (~1.81). Neither produces a readable shadow edge. Use thermal testing or SG instead.
- Assuming the scratch test is reliable. While diamond is harder (10 vs 8-8.5), hardness testing is destructive and impractical for finished stones. It also does not distinguish diamond from other hard simulants.
- Confusing CZ's lack of inclusions with perfection. CZ is typically flawless because it is manufactured. A flawless "diamond" should raise suspicion — most natural diamonds contain inclusions visible under 10x.
Frequently Asked Questions
Can a diamond tester tell diamond from cubic zirconia?
Yes. Unlike moissanite, cubic zirconia has low thermal conductivity and reads "simulant" on any standard thermal diamond tester. This is the fastest and most common separation method in practice.
Why does CZ look more "sparkly" than diamond?
CZ has higher dispersion (0.066) than diamond (0.044), which produces more spectral fire — rainbow flashes of color. To a trained eye, CZ's fire appears excessive and is a visual clue. However, this is subjective and not a substitute for instrumental testing.
Is CZ always synthetic?
Yes. All cubic zirconia on the market is lab-created (synthetic ZrO2). Natural ZrO2 (baddeleyite) is monoclinic, not cubic, and has never been found in gem quality.
Identifying a colorless stone? GemID walks you through thermal testing and SG measurement step by step.
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