Real or fake diamond? Recognising cubic zirconia and moissanite
Two stones are most often mistaken for diamond: cubic zirconia and moissanite. With some practice you can often recognise cubic zirconia yourself, moissanite hardly ever. What home tests are worth, and which ones to leave alone.
Cubic zirconia, moissanite and diamond: what is the difference?
- Diamond is pure carbon and the hardest natural material: 10 on the Mohs scale. It conducts heat exceptionally well.
- Cubic zirconia (cubic zirconium oxide) is a cheap synthetic stone, hardness about 8. It is noticeably heavier than diamond and conducts heat poorly.
- Moissanite (silicon carbide) is a synthetic stone with a hardness of about 9.25. It conducts heat almost as well as diamond and gives off more rainbow-coloured fire.
What can you check at home?
No home test gives certainty, but together they point in a direction:
- The breath test. Breathe on the stone. On diamond the condensation clears almost immediately; on cubic zirconia it stays longer.
- Wear. After years of wear, cubic zirconia gets rounded, dull facet edges and scratches. Diamond stays sharp.
- Marks and papers. A mark such as 'CZ' inside the ring points to cubic zirconia. A laboratory report, or a laser inscription on the girdle, points to a graded stone.
- The setting. A large stone in silver or in low-grade gold is more often an imitation, but that is no rule.
Do not do a scratch test or a heat test. Dragging a stone across glass or heating it can damage a diamond, and the setting too. A 'genuine' result from such a test proves little anyway.
Why a diamond tester lets moissanite through
An ordinary diamond tester measures how quickly a stone draws away heat. Cubic zirconia and glass conduct poorly and fail. Moissanite conducts heat almost as well as diamond, so such a tester often reads it as 'diamond'.
There are testers that also measure electrical conductivity; moissanite conducts electricity, most diamonds do not. That is not a final verdict either. A lab-grown diamond passes both tests as diamond, because it is diamond.
How gemmologists establish it with certainty
Our gemmologists look first with a loupe and microscope. Moissanite is doubly refractive: look through the stone at an angle and you see the facet edges at the back doubled. Diamond and cubic zirconia are singly refractive. They also compare weight and measurements, because cubic zirconia weighs clearly more at the same size.
With specialised equipment they then test whether a diamond is natural or grown. The assessment is free and without obligation; for diamonds we schedule an appointment.
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Questions about Real or fake diamond
How do I know whether my diamond is real?
At home you only get indications: condensation that clears quickly from diamond, sharp facet edges, a mark or a report. Only a gemmologist with loupe, microscope and measuring equipment gives certainty.
Does a diamond tester recognise moissanite?
An ordinary heat tester usually does not, because moissanite conducts heat almost as well as diamond. A tester that also measures electrical conductivity can identify moissanite. Under magnification moissanite shows itself through doubled facet edges.
Is cubic zirconia worth anything?
Cubic zirconia is a cheap synthetic stone and has no resale value. The gold or silver of the piece does have material value, and that is what we pay for.
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Sources
Where you can verify what you have just read.
- LBMAThe London gold and silver price our daily rate is built on
- WaarborgHollandThe Dutch assay office that verifies fineness and strikes hallmarks
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