Lab-Grown Diamonds vs Cubic Zirconia

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A lab-grown diamond is a real, permanent diamond; cubic zirconia (CZ) is an inexpensive lab-made simulant that looks sparkly at first but clouds and scratches over time. They sit at opposite ends of the spectrum — not two grades of the same thing, but two different materials that happen to be transparent and colourless. Here is the honest comparison, including the measurable properties, how to tell them apart, and the cases where CZ is genuinely the right choice.

Key takeaways

• A lab-grown diamond is a real diamond (pure carbon, Mohs 10); CZ is a simulant (zirconium dioxide, Mohs 8.5).
• The Mohs scale is ordinal, not linear — the gap between 8.5 and 10 is far larger than it sounds.
• CZ throws more rainbow fire than diamond and can look glassy or 'too perfect'.
• CZ is about 1.7 times denser than diamond, so the same size stone is noticeably heavier in the hand.
• CZ scratches and clouds with everyday wear, and is designed to be replaced rather than kept.
• CZ is not graded or certified — there is no 4Cs report for a simulant.
• For a forever ring, a lab-grown diamond is the lasting choice; CZ genuinely suits costume, travel and placeholder pieces.

What is cubic zirconia?

Cubic zirconia is a man-made crystal — zirconium dioxide, stabilised into a cubic structure — created as an inexpensive diamond simulant. A simulant is any material that imitates the appearance of a diamond without sharing its chemistry. That is the whole distinction: CZ is designed to look like a diamond, whereas a lab-grown diamond simply is one. CZ is colourless, very sparkly when new, and extremely affordable, which makes it the standard choice for fashion jewellery, travel pieces and stand-ins. It is not a fraud or a trick when sold honestly; it is a different product with a different job.

Where cubic zirconia came from

CZ is a genuine feat of materials science, and the story is more interesting than its reputation suggests. Zirconium dioxide melts at roughly 2,750°C — hotter than any crucible that could hold it. Soviet scientists at the Lebedev Physical Institute in Moscow, working under V. V. Osiko, solved the problem in the early 1970s with the skull crucible: the powder is held in a water-cooled copper vessel and heated from the inside by radio-frequency induction, so the outer layer stays solid and forms a shell of the material's own substance. The melt is, in effect, contained by itself.

Their work was published in 1973 and commercial production began in 1976. They called the result Fianit, after the institute's initials — a name that never travelled outside the USSR. By 1980, world production had reached around 50 million carats a year. Understanding this helps set expectations honestly: CZ was never a cheap fake thrown together to fool people. It was engineered to be an excellent, affordable colourless stone, and it succeeds at that. It was simply never engineered to survive fifty years on a hand.

What is a lab-grown diamond?

A lab-grown diamond is chemically, physically and optically identical to a mined diamond — pure carbon in the same crystal structure, the hardest material there is, and graded on the 4Cs by an independent laboratory. It is a real diamond that formed in a growth chamber rather than in the earth, and no amount of wear, heat or time turns it into anything else. Note that IGI still grades lab-grown diamonds on the full 4C scales, while GIA has used a two-tier Premium or Standard classification for them since 1 October 2025. See how they are made, lab-grown vs mined, and the 2025 GIA grading change.

Side by side: the measurable differences

These are not matters of opinion. Every figure below is a standard gemological constant, and they are what a laboratory uses to tell the two apart in seconds.

Property	Lab-grown diamond	Cubic zirconia
What it is	Real diamond — carbon	Simulant — zirconium dioxide
Hardness (Mohs)	10 — the hardest material known	8.5
Refractive index	2.42	2.18
Dispersion ('fire')	0.044	0.058–0.066 — noticeably higher
Specific gravity	3.52	5.6–6.0 — about 1.7× heavier
Durability	A lifetime; never clouds	Scratches and clouds over time
Look	Balanced white and coloured brilliance	Glassy, heavy rainbow fire
Certification	IGI or GIA report	None — simulants are not graded
Resale	Retains standing as a real diamond	Effectively none

Why 8.5 is not 'nearly 10'

The single most misleading number in this comparison is the hardness figure, because the Mohs scale is a ranking, not a measurement. It records only which material scratches which — not by how much. The steps are wildly uneven, and the last one is the largest on the entire scale: the jump from 9 to 10 is far greater than the jump from 1 to 9. So a stone at 8.5 is not 85% as scratch-resistant as a diamond; it is in a different regime altogether.

This matters in a specific, practical way. Ordinary household dust contains quartz at Mohs 7. Quartz cannot touch a diamond, but it is close enough to CZ that everyday contact — wiping, washing, a hand in a pocket — slowly abrades the facet edges and the polish. That is the mechanism behind the clouding, and it is why the effect appears within a few years of daily wear rather than decades.

How they age

This is the real difference. A diamond — mined or lab-grown — stays brilliant permanently, because nothing in daily life is hard enough to mark it and it is chemically inert. Cubic zirconia is softer and more porous, so it accumulates fine surface scratches and absorbs oils, gradually turning cloudy or grey. The facet edges soften first, which is what actually kills the sparkle: crisp edges are what split light into brilliance. CZ jewellery is meant to be replaced on a cycle; a diamond ring is meant to be kept and handed on.

The look

New CZ sparkles, and at arm's length it is convincing. Up close it reads differently, for a reason the numbers above predict. Its dispersion is around 0.058–0.066 against diamond's 0.044, so it throws considerably more coloured 'fire' — an excess of rainbow flashes where a diamond gives a balance of white light and colour. Its lower refractive index means less light is returned overall, which is why CZ can look glassy or slightly watery rather than bright. It is also flawless in a way real stones rarely are, and that perfection is itself a tell.

How to tell them apart

Several of the differences are noticeable without equipment, though none is conclusive on its own.

• Weight. The most reliable everyday tell. CZ is about 1.7 times denser, so a CZ cut to the same dimensions as a three-carat diamond feels distinctly heavy for its size.
• Fire. Excessive rainbow flash relative to white brilliance suggests a simulant.
• Wear. Look at the facet edges under a loupe. Rounded, softened edges and a hazy surface point to a softer material that has been worn.
• Condition. A stone with no inclusions whatsoever, in a piece that is clearly not new, is worth a second look.

What settles it is a laboratory report. A simulant cannot be graded on the 4Cs because it is not a diamond, so the absence of certification is itself informative — and it is the reason we specify an IGI report on every stone rather than asking anyone to take our word for it. Our guide to reading a diamond certificate covers what to look for on the document itself.

When cubic zirconia is genuinely the right call

It would be dishonest to pretend CZ has no place. There are situations where it is the sensible choice, and recommending a diamond in all of them would be selling rather than advising.

• Travel. A stand-in worn abroad or somewhere you would rather not risk the real ring.
• A proposal placeholder. Some couples propose with an inexpensive stone precisely so they can choose the real ring together afterwards — a genuinely good idea, and one we see often.
• Costume and fashion pieces. Statement jewellery worn a handful of times, where longevity is beside the point.
• Trying a look. Living with a shape or a setting silhouette before committing to it.

What CZ cannot do is be the stone in a ring meant to last a lifetime. That is not snobbery — it is the material behaving exactly as designed.

Which should you choose?

For a costume piece, a travel stand-in, or a temporary placeholder, CZ is perfectly sensible. For an engagement ring meant to last a lifetime, a lab-grown diamond is a real diamond at a fraction of mined prices, and it will look as brilliant in fifty years as it does today. The comparison people should actually be weighing is not diamond against simulant but lab-grown against mined, where the material is identical and only the origin and the price differ.

Montare works only in real, certified lab-grown diamonds, held to a fixed floor: D–E colour, VVS1–VVS2 clarity, Triple Excellent cut with table and depth in the ideal range for the shape, three carats minimum, IGI certified. You describe what you are looking for and we source the stone that clears it; our goldsmiths then cast, set and polish the ring by hand in Toronto. The full per-shape figures are on our standards. If you are also weighing moissanite — a far more serious contender than CZ — we compare it in lab-grown diamonds vs moissanite.

Frequently asked questions

Is cubic zirconia a diamond?

No. CZ is a simulant made of zirconium dioxide — a different material engineered to look like a diamond. A lab-grown diamond, by contrast, is a real diamond: pure carbon, chemically and optically identical to a mined stone.

Does cubic zirconia last?

Not like a diamond. CZ is softer and tends to scratch and cloud within a few years of daily wear, so it is usually replaced rather than kept.

Can you tell cubic zirconia from a diamond?

Often, yes — CZ throws heavier rainbow fire and can look glassy, it feels noticeably heavy for its size, and over time it clouds. A diamond stays clear and balanced. A laboratory report settles it conclusively.

Is lab-grown diamond worth it over CZ?

For a lasting ring, yes — a lab-grown diamond is a real, permanent diamond at far below mined prices. CZ is best for temporary or costume pieces.

Why does cubic zirconia go cloudy?

At Mohs 8.5 it is soft enough to be abraded by ordinary household dust, which contains quartz at Mohs 7. Fine scratches accumulate and the crisp facet edges that create sparkle gradually soften, while the porous surface holds oils. A diamond at Mohs 10 is unaffected by both.

Is cubic zirconia heavier than a diamond?

Yes — considerably. CZ has a specific gravity of 5.6 to 6.0 against a diamond's 3.52, so a CZ cut to the same dimensions weighs roughly 1.7 times as much. It is one of the most reliable ways to tell them apart by hand.

Can cubic zirconia be certified or graded?

No. The 4Cs are a diamond grading system, so laboratories do not issue clarity, colour and cut grades for simulants. A stone sold with an IGI or GIA diamond report is a diamond; the absence of one on a colourless stone is worth asking about.

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