Diamond vs Cubic Zirconia: How to Tell Them Apart

Last updated: April 2026

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
Diamond vs Cubic Zirconia: where the ranges part Where the ranges part Diamond and Cubic Zirconia, each recorded property on an axis scaled to the pair Diamond Cubic Zirconia Refractive index not readable both over the 1.81 refractometer limit — no shadow edge to read for either; the ranges are from other methods 2.417 2.150–2.180 2.138 2.284 2.429 Specific gravity separates no overlap — a gap of 2.07, wider than the ±0.10 a reading is trusted to 3.50–3.53 5.60–6.00 3.39 4.75 6.11 Ranges as recorded in gems.json for Diamond and Cubic Zirconia; every axis is scaled to the pair, so read the tick labels. A gap wider than the instrument's tolerance (RI ±0.010, SG ±0.10, birefringence ±0.003, as the identification engine applies them) means a single clean reading settles it; an overlap, or a gap inside that tolerance, means that instrument cannot, on its own. Hardness is not drawn: a scratch test damages a cut stone. A refractometer has no shadow edge above about 1.81, so an RI marked over the limit is read by another method. Birefringence is a single recorded value per material.

The Definitive Tests

  1. 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.)
  2. 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.
  3. 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.
  4. 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

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? Loupewise walks you through thermal testing and SG measurement step by step.

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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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