The short answer fits in one word: it depends. A fake diamond can be a cubic zirconia, a moissanite, a lead glass or a white topaz. These are different materials, with different names and different behaviours. Two errors circulate everywhere on this subject, jewellers included. The first files the laboratory-grown diamond on the fake side, which is inaccurate. The second confuses zircon and zirconia, which have nothing to do with each other. This article corrects both and gives the real hardness figures.

Simulant, laboratory-grown diamond, mined diamond: three families

Before naming anything, three separate boxes have to be set out. Most French pages set out only two, and that is the source of all the confusion.

Box 1, the simulant. A material different from diamond, chosen because it looks like it. Different chemical composition. Different hardness. Different optical behaviour. Cubic zirconia, moissanite, glass and rhinestone sit in this box. These are the real fake diamonds.

Box 2, the laboratory-grown diamond. Crystallised carbon, produced by people instead of being mined from the ground. Same chemical composition. Same crystal structure. Same hardness 10. Same refractive index. It is not a simulant, it is a diamond obtained another way.

Box 3, the mined diamond. The same crystallised carbon, formed underground a very long time ago.

Wikipedia and a good part of the French SERP mix boxes 1 and 2 under the word "synthetic". That is understandable linguistically and wrong gemmologically. A simulant looks like a diamond. A laboratory-grown diamond is a diamond. The difference lies in the process of formation, not in the nature of the material.

We sell laboratory-grown diamonds, so we are not neutral. Check elsewhere. You will find the same distinction at every gemmological laboratory. A laboratory-grown diamond must be named in explicit words on an invoice, as a laboratory-grown diamond. A cubic zirconia is named "cubic zirconia" and never gets to borrow the word diamond.

Cubic zirconia, also called zirconium oxide

It is the most frequent answer to the question "what is a fake diamond called". Cubic zirconia is a stabilised zirconium oxide, produced in a laboratory since the 1970s.

Its characteristics give it away. Its hardness is 8 to 8.5 on the Mohs scale, against 10 for diamond. Its refractive index is around 2.15, against 2.42 for diamond. Its density is far higher: at equal size, a cubic zirconia weighs about 1.7 times the weight of a diamond.

To the eye, a new cubic zirconia is very convincing. It is even sometimes too perfect: the natural inclusions are missing. Its fire is slightly more coloured and more dispersed than that of a diamond.

It is found massively as a diamond imitation on 925 silver. It is an honest material as long as it is named correctly. The problem is never the cubic zirconia, it is the seller who writes "diamond" on the label.

Zircon or zirconia: the most widespread mistake

Here is the point where almost every French page gets it wrong, well ranked jewellery sites included.

Zircon is a natural mineral. It is a zirconium silicate, mined from the ground, present in rocks for billions of years. Its hardness sits between 6 and 7.5 depending on the variety. It exists colourless, but also blue, brown and yellow. It is a gemstone in its own right, with a fine dispersion.

Zirconia, or cubic zirconia, or zirconium oxide, is a synthetic material. It barely exists in nature. It is manufactured to imitate diamond. Its hardness is 8 to 8.5.

Two close words, two unrelated materials. One is a natural silicate, the other an artificial oxide. Writing "zircon" when speaking of an imitation stone is a mistake, even if everyone makes it. And giving zircon a hardness of 8, as one often reads, mixes the two up.

Keep the marker in mind: zircon, ending in n, a natural mineral. Zirconia, ending in a, a synthetic imitation.

Moissanite, the most convincing simulant

Moissanite is a silicon carbide. The natural mineral exists, discovered in a meteorite, but it is infinitely rare. Commercial moissanite is produced in a laboratory.

It is the substitute hardest to tell apart from a diamond. Its hardness reaches 9.25 on the Mohs scale, just behind diamond. It takes daily wear very well, far better than a cubic zirconia.

Its weak point is its excess. Its refractive index is higher than that of diamond, around 2.65, and its dispersion is more than twice as strong. The result: moissanite throws back very marked rainbow flashes, which professionals sometimes call a "disco" fire. On a large stone, the effect is obvious.

Moissanite is also birefringent, which diamond is not. Seen through a loupe at a certain angle, you can make out a doubling of the edges of the pavilion facets.

Is it a fake diamond? Yes, strictly speaking: it is not carbon, it is another material that looks like diamond. But it is an interesting gem in its own right, provided it is sold under its own name.

Glass, rhinestone and lead crystal

The bottom of the simulant range deserves a few lines, because the words there are vague.

Plain glass has a hardness of 5 to 6. It scratches immediately and its lustre is soft.

Rhinestone is a cut lead glass, often given a reflective coating on the back. Adding lead raises the refractive index and therefore the sparkle. The hardness stays low, around 6.

Commercial "crystal" is not a crystal in the mineralogical sense. It is a lead glass. The term is an industrial usage, not a gemstone category.

These materials are quickly recognised. They warm up on contact and their edges blunt after a few months of wear.

White topaz and white sapphire, the natural fake diamonds

It is often forgotten that some simulants are perfectly respectable natural stones, simply used in place of diamond.

White topaz shows a hardness of 8. It is colourless and transparent. Its flaw is optical: its refractive index is low, around 1.62. It lacks brilliance and looks glassy next to a diamond.

White sapphire reaches hardness 9. It therefore holds up very well day to day. But its dispersion is low, which means little fire. It gives a white, quiet lustre, without the coloured flashes of diamond. Many customers find it slightly milky.

Colourless zircon, for its part, has a fine dispersion, close to that of diamond. Its weakness is hardness, between 6 and 7.5, and a tendency for the edges to chip.

What a simulant becomes after three years

The hardness table means nothing until it is translated into years of wear. Let us do that.

A cubic zirconia worn as a daily ring loses its polish in one to two years. The facet edges round off first, because they take all the friction. The table micro scratches. The stone turns milky and its fire fades away. It is reversible by repolishing, but repolishing often costs more than the stone.

Glass or rhinestone goes faster. Count a few months to see a matt veil appear.

Moissanite holds up very well. At 9.25, it resists daily wear. It stays identifiable by its excess of fire, which does not fade.

A diamond, laboratory-grown or mined, does not scratch in normal use. Only another diamond can scratch it. That is the whole meaning of hardness 10. It does not make it unbreakable: a violent knock on a corner can chip it. Our jewellery care guide sets out the right habits.

Reading a label and an invoice in France

The most concrete point of this article, and the one the SERP does not offer.

In France, the gemstone trade regulates the designations. The word "diamond" on its own is reserved for the mined diamond. A stone produced in a laboratory must be designated by an explicit term, such as "laboratory-grown diamond" or "lab-grown diamond". A simulant must carry its own name, cubic zirconia or moissanite, without ever borrowing the word diamond.

Three useful reflexes before buying.

First reflex: look for the exact name of the material on the product page. A site that writes only "white stone" or "crystal" does not want you to know.

Second reflex: check that an independent laboratory report is present. At Lauredi, the stones are IGI certified. A report carries a number you can verify online.

Third reflex: insist that the invoice repeats the designation. The invoice is the document that counts in a dispute or for your insurance. We set these points out in our article on buying jewellery online.

What to remember

A fake diamond carries a precise name, and that name depends on the material. Cubic zirconia, hardness 8 to 8.5. Moissanite, 9.25. Glass and rhinestone, 5 to 6. White topaz, 8. White sapphire, 9. None of them is carbon.

The laboratory-grown diamond, for its part, is crystallised carbon, at hardness 10. Filing it among the fakes is a category error. It must be named in explicit words on an invoice, as a laboratory-grown diamond, and that is exactly what the regulation says.

And stop writing zircon when you mean zirconia. One is a natural mineral, the other a manufactured imitation. You will know immediately who, across the counter, has mastered the subject. To go further, read our comparison real versus fake diamonds.

Read next. A laboratory-grown diamond is not a simulant: our article how to identify a laboratory-grown diamond explains why and how to check it. For the names of real coloured stones, see the names of coloured stones. And to compare budgets, our table on the price of a coloured diamond puts the figures side by side.

FAQ

What is a fake diamond called?

Most often, a cubic zirconia, also named zirconium oxide. The other common simulants are moissanite, the lead glass called rhinestone, white topaz and white sapphire. Each carries its own name and none of them may be sold under the word diamond.

Is a laboratory-grown diamond a fake diamond?

No. It is crystallised carbon, with the same structure, the same hardness 10 and the same optical properties as a mined diamond. Only the mode of formation differs. On a French invoice it is designated in explicit words, as a laboratory-grown diamond.

What is the difference between zircon and zirconia?

Zircon is a natural mineral, a zirconium silicate, of hardness 6 to 7.5. Zirconia, or cubic zirconia, is a zirconium oxide manufactured in a laboratory to imitate diamond, of hardness 8 to 8.5. The two words look alike, the materials have no connection at all.

Is moissanite a fake diamond?

Yes, in the sense that it is not diamond but silicon carbide. It is however a solid stone, of hardness 9.25, suited to daily wear. It is spotted by its very coloured fire, clearly more dispersive than that of a diamond.

Does the fog test identify a fake diamond?

It gives an indication, not a proof. A diamond clears fog very fast thanks to its thermal conductivity, a cubic zirconia holds it longer. But moissanite behaves like a diamond on this test, and ambient humidity distorts the result.

How much does a fake diamond cost per carat?

A cubic zirconia costs a few euros per stone. Moissanite sits far above, often several hundred euros per carat. The price gap between these two simulants is far wider than most buyers imagine.

Does a seller have the right to write "diamond" for a cubic zirconia?

No. In France, the designations of gemstones are regulated. The word diamond on its own is reserved for the mined diamond, a grown stone must be explicitly qualified, and a simulant must carry its own name. A product page that stays vague about the nature of the stone is a warning sign.

Fiona