Diamond, Moissanite or Cubic Zirconia? How to Tell Them Apart
They can look alike across a room, but only one is diamond. How lab-grown diamonds, moissanite and cubic zirconia differ, and how to tell them apart without being fooled.
Across a dinner table, a diamond, a moissanite and a cubic zirconia can look surprisingly similar. Up close, and over years of wear, they are very different. Here is how they compare, and how to make sure you are getting what you pay for.
Diamond (natural or lab-grown)
Pure carbon, hardness 10, with a balance of white brilliance and coloured fire. Lab-grown and natural diamonds are the same material; only their origin differs. They keep their polish and sharp facet edges for generations.
Moissanite
Silicon carbide, almost always lab-created for jewellery. It is very hard (9.25) and very sparkly, with more rainbow fire than diamond, which some people love and others find a little "disco ball" in sunlight. It is a durable, legitimate gemstone, but it is not a diamond and should never be sold as one.
Cubic zirconia (CZ)
A man-made crystal that is much softer (8–8.5) and heavier than diamond. New CZ can look bright, but it scratches and its edges round over time, and it tends to look glassy or cloudy after a few years of daily wear. It is inexpensive costume and fashion jewellery material.
How to tell them apart
- Look at the fire: strong rainbow flashes everywhere suggest moissanite.
- Check facet edges: rounded, scratched edges on an older stone suggest CZ.
- Doubling: under a loupe, moissanite often shows doubled facet edges (it is doubly refractive); diamond does not.
- Testers: basic thermal testers can mistake moissanite for diamond; multi-testers and gem labs can tell them apart.
- Paperwork: a diamond grading report from an independent lab, matching the laser inscription, is the real proof.
The numbers side by side
| Diamond | Moissanite | Cubic zirconia | |
|---|---|---|---|
| Material | Carbon | Silicon carbide | Zirconium dioxide |
| Hardness (Mohs) | 10 | 9.25 | 8–8.5 |
| Refractive index | About 2.42 | About 2.65–2.69 | About 2.15–2.18 |
| Fire (dispersion) | 0.044 | 0.104 | About 0.060 |
| Density (g/cm³) | 3.52 | 3.22 | About 5.6–6.0 |
Moissanite's dispersion is more than twice diamond's, which is where its strong rainbow flashes come from. Cubic zirconia's density means a CZ weighs far more than a diamond of the same size.
Other look-alikes you may meet
White sapphire
Hard (9) and durable, but with much less sparkle; it looks milky quickly when not cleaned.
White topaz
Hardness 8, glassy and soft-looking; fine for occasional jewellery.
Crystal and glass
Lead glass, often called crystal. Bright when new, but soft and quick to scratch.
YAG and GGG
Synthetic garnets used as diamond simulants before cubic zirconia, still found in vintage pieces.
Real diamonds, cut in our workshop
Is moissanite a fake diamond?
It is not diamond, but it is a real gemstone in its own right. Problems arise only when it is sold as diamond.
Are lab-grown diamonds the same as moissanite?
No. Lab-grown diamonds are carbon, exactly like mined diamonds; moissanite is silicon carbide.
Can a diamond tester tell moissanite from diamond?
Basic thermal testers often cannot. Multi-testers that also measure electrical conductivity, or a gem lab, can.
Is white sapphire a good diamond alternative?
It is durable, but it has far less brilliance and fire than diamond and looks dull quickly when it gets oily.
Does cubic zirconia weigh more than diamond?
Yes, about 1.6 to 1.7 times as much for the same size.
