Cubic zirconia is the most common diamond simulant on the market, and at arm's length under bright lights it can fool almost anyone. The problem is that a CZ is not diamond-adjacent in durability, value or optical character — it is a coloured glass-like crystal that looks convincing for a few months and then scratches, clouds and dulls. If you are buying a diamond or a high-grade lab-grown stone, knowing how to tell the two apart protects you from accidentally receiving a CZ when you paid for the real thing.

This guide walks through the simple at-home tests that separate diamond from cubic zirconia, and explains why none of them are a substitute for a proper grading report. If you want the broader picture of simulants, our moissanite versus diamond guide covers the other common alternative.

The single biggest difference: hardness

Diamond is the hardest known natural material, scoring 10 on the Mohs scale, and it cannot be scratched by anything except another diamond. Cubic zirconia scores around 8 to 8.5, which sounds close but is not: after a year of being thrown in a jewellery drawer, a CZ is covered in fine scratches that make it look frosted, while a diamond still looks like new. This is the reason a CZ ring never ages well, no matter how good it looked on day one. Our durability guide explains where moissanite sits between the two.

The breath test (fog test)

Breathe on the stone as you would on a mirror. A real diamond disperses heat so quickly that the fog disappears almost instantly. A cubic zirconia holds the heat, so the fog lingers for two to four seconds. This is the most famous home test, but it is not perfect: a very small diamond may also fog briefly, and a mounted stone fogs differently from a loose one. Use it as a quick signal, not as a verdict.

The weight test

CZ is denser than diamond. A 1-carat CZ and a 1-carat diamond are both sold by weight, but because CZ is heavier, a CZ of the same visual size weighs more. If you have a known diamond of the same size to compare against, a quick feel in the fingers is telling. Even better, a CZ of the same millimetre size as a 6.5mm diamond will weigh noticeably more on a jeweller's scale. This is why reputable sellers always list millimetre sizes, as explained in our carat size guide.

Look at the edges and the back

Diamond is hard enough that its facets stay sharp and crisp even after years of wear. A CZ scratches and rounds its edges quickly, and under magnification the facet junctions look soft rather than sharp. Look through the back of a mounted stone: diamond is usually open-backed so light can pass through, while cheap CZ rings are often foil-backed to fake brilliance, and the foil tarnishes after a year. If the back of the stone looks sealed and mirrored, be suspicious.

Light return and fire

Diamond returns white light (brilliance) in a balanced, controlled way. CZ overdoes it: it throws too much rainbow "fire" and looks almost glittery under office lighting. Moissanite also throws more fire than diamond, which is why CZ and moissanite are sometimes confused — but moissanite is far harder and more durable than CZ. Our moissanite fire guide explains that rainbow flash in detail.

Price and provenance give it away

A 1-carat diamond, even lab-grown, costs real money. If a listing promises a 2-carat "diamond" for under £50, it is a CZ, full stop. The price gap is not a scam waiting to be negotiated; it is the physical difference between two materials. Always check the certificate: a real diamond sold online comes with a GIA or IGI report, and a moissanite with a GRA one. Our certificate reading guide shows what to look for on the report itself.

When CZ is actually the right choice

None of this means CZ is useless. It is a sensible choice for a costume piece, a travel spare, or a ring you will only wear a handful of times. It is not the right choice for an engagement ring that is meant to last decades. If budget is the reason you are tempted by CZ, a lab-grown diamond or a moissanite gives you the durability of a real gem at a fraction of a mined-stone price — our lab-grown versus mined guide lays out those options honestly.

The bottom line

A CZ looks great for a few months and then scratches, clouds and loses its fire. A diamond — mined or lab-grown — is forever. If you want the real thing at a price, buy a certified stone from a seller who lists millimetres and a report, and run the breath and weight tests before you commit. If you are ever unsure about a stone you already own, any independent jeweller will tell you within a minute, usually for free.

What about moissanite and white sapphire?

Moissanite is often mistaken for CZ, but it is a world away in durability. It scores 9.25 on the Mohs scale, returns strong fire, and is sold with a certificate — our moissanite versus diamond comparison explains that gap in detail. White sapphire is a natural gem that is softer than moissanite but still a real stone, with a quiet, milky look rather than a glittery one. If you are choosing between these three simulants, our side-by-side guide on moissanite, white sapphire and CZ lays out the differences.

Frequently asked questions

Does a CZ pass the breath test on its own? No. The breath test points toward diamond, but only a certificate confirms it. CZ fogs longer, but very small diamonds can also fog briefly.

Can a jeweller tell CZ from diamond instantly? Yes, with a thermal conductivity tester, which distinguishes diamond from almost every simulant in seconds. This is the definitive test.

Is a CZ ring worth anything? As jewellery it has the value of its metal setting, but the stone itself has almost no resale value.

How long does a CZ ring last? It looks new for a few months, then scratches and clouds. With careful storage it may look acceptable for a year or two, but it will never keep the polish of a diamond.

The honest bottom line

If a stone is sold as a diamond and priced like a trinket, treat it as CZ. If you want a real gem at an honest price, choose a certified lab-grown diamond or a GRA moissanite rather than gambling on an uncertified "diamond" that arrives in a plain pouch. The tests above are quick checks, but the certificate is the proof. Our reading a diamond certificate guide is the last step before you buy, and it takes five minutes.