Cubic zirconia, lab-grown or natural?

Looking for diamond, finding something else

In the 1890s, the American inventor Edward Acheson was trying to make diamond. He passed an electric arc through a mixture of carbon and molten clay, and found a hard new crystal in the result. It wasn't diamond. He called it carborundum, and it became one of the great industrial abrasives.

In 1904 the Nobel prize-winning French chemist Henri Moissan described the same material, silicon carbide, occurring in nature, in the Canyon Diablo meteorite from Arizona. The gem expert George Frederick Kunz named the mineral moissanite in his honour. Nearly a century after that, it reappeared as a near-colourless gem, one of the most convincing diamond look-alikes ever made.

Acheson's search for man-made diamond had found an imitation instead. Today, both exist side by side: true diamonds grown by people, and look-alikes that are something else entirely.

Three kinds of stone

• Natural diamond. A mineral made of carbon, formed by nature. In the trade guideline used in the UK, "diamond" on its own always means a natural diamond.
• Laboratory-grown diamond. Also called synthetic or laboratory-created diamond. A man-made product with essentially the same chemical composition, crystal structure and physical properties as natural diamond. As GIA puts it, these are not imitations: they are real diamonds.
• Simulant. Also called an imitation. A material, natural or man-made, that looks like diamond without having its composition or properties. Cubic zirconia and moissanite are the best known; white sapphire and yttrium aluminium garnet are others.

The key point: a laboratory-grown diamond is the same material as a mined one, made by a different route. Cubic zirconia is a different material altogether.

How diamonds are grown

People have been growing diamond for longer than most of us realise. Union Carbide produced the first diamonds by chemical vapour deposition in 1952, General Electric researchers made the first gem-quality laboratory-grown diamonds in the 1970s, and GIA published its first scientific study of them in 1971. Two methods are used.

• HPHT: high pressure, high temperature. A capsule holding a source of carbon and a mixture of metals is heated to 1,300 to 1,600°C under pressures of 5 to 6 gigapascals, about what a diamond would meet 150 to 190 kilometres underground. The carbon dissolves in the molten metal and crystallises as diamond on a seed.
• CVD: chemical vapour deposition. Quite unlike nature, this happens at near-vacuum pressure. A chemical reaction in a chamber releases carbon atoms, which settle on thin diamond seed plates and build up the crystal layer by layer.

Either way, the result has the same crystal structure as natural diamond, with nearly the same chemical, optical and physical properties.

The look-alikes: CZ and moissanite

Cubic zirconia is zirconium dioxide, grown by skull melting and on sale as a diamond imitation since 1976. It has slightly more fire than diamond but less brilliance, and it is much heavier for its size. Our guide to what cubic zirconia is covers it in full.

Synthetic moissanite is silicon carbide. In 1997, GIA's journal Gems & Gemology described near-colourless moissanite as being closer to diamond in overall appearance and heft than any previous imitation. It is hard, at 9¼ on the Mohs scale, with excellent toughness. Its brilliance is only slightly less than diamond's, and it can show a slightly greenish or greyish tint.

One property gives moissanite away. It is doubly refractive: light entering it splits in two, so the edges of the back facets look doubled under a loupe, and sometimes even to the naked eye in a larger stone.

Side by side

These are GIA's figures for colourless stones.

Moissanite's two refractive index figures are the doubling in numbers: light travels through it at two different speeds. Its dispersion is more than twice diamond's, which is why it can flash with strong rainbow colours.

How each is identified

Simulants against diamond. A trained gemmologist can separate CZ and moissanite from diamond with standard tests. CZ gives itself away by its weight in a loose stone, and a jeweller's thermal tester tells it from diamond. Moissanite is trickier: it conducts heat so much like diamond that thermal testers read it as "diamond". Its doubled facet edges under magnification are the giveaway.

Laboratory-grown against natural. This is a different problem, because both are diamond. GIA is blunt: you can't spot a laboratory-grown diamond by looking at it through a loupe, and the only way to tell is laboratory testing. The difference lies in how the crystal grew and what it went through afterwards, which leaves traces in its defects and impurities. Laboratories read those traces with spectroscopic instruments, and can usually say whether a grown diamond was made by HPHT or CVD.

What they may be called

Because the stones look so alike, the words matter. The rules are slightly different in the United States and the UK.

United States: the FTC Jewelry Guides

The Federal Trade Commission approved revised Jewelry Guides on 24 July 2018. Among the changes, it removed the word "natural" from its definition of diamond. The definition now describes a mineral consisting essentially of pure carbon, crystallised in the isometric, or cubic, system, with a hardness of 10. In practice:

• A laboratory-grown diamond can be called a diamond, but only with a clear qualifier such as "laboratory-grown", "laboratory-created" or "[manufacturer name]-created".
• "Cultured" may be used for a laboratory-grown diamond only if it comes with one of those clear disclosures.
• Words such as "laboratory-grown", "laboratory-created" or "synthetic" may only go with a gem's name if the product really has essentially the same properties as that gem. A CZ can't be sold as a "laboratory-grown diamond".
• Imitations must be qualified with words such as "imitation" or "simulated". The Guides say "faux" isn't enough on its own.

United Kingdom: the trade's terminology guideline

In the UK, the National Association of Jewellers publishes a Diamond Terminology Guideline, endorsed by nine international diamond and jewellery bodies, including CIBJO, the World Jewellery Confederation. It is built on the international standard ISO 18323 and CIBJO's Diamond Blue Book. It says:

• "Diamond" always means a natural diamond. Use "natural diamond" only where it helps to make the difference clear.
• For grown diamonds, use "synthetic", "laboratory-grown" or "laboratory-created".
• Don't shorten these to "lab-grown" or "lab-created".
• Don't call grown diamonds "cultured" or "cultivated", because those words belong to organic products such as pearls.
• Don't describe grown diamonds as "real", "genuine", "precious", "authentic" or "natural".

Lab-grown coloured stones and birthstones

Diamond was not the first gem to be grown. That honour belongs to ruby: in 1902 Auguste Verneuil announced his flame-fusion process, which drops powdered chemicals through a very hot flame to build a crystal. It remains the cheapest and most common way to make synthetic corundum, the mineral of ruby and sapphire, and spinel.

A laboratory-grown gem shares virtually all the chemical, optical and physical characteristics of the natural one. Synthetic opal and turquoise are the exceptions GIA names, because they can contain extra compounds. Several birthstones are commonly made this way.

The same naming rules apply. Under the FTC Guides, a laboratory-grown ruby must be described with a qualifier such as "laboratory-grown" or "laboratory-created", and so must the word "birthstone" itself. A red glass imitation can't be called ruby at all, only an imitation or simulated ruby.

For any important ruby, sapphire or emerald, the National Association of Jewellers advises getting a gemstone identification report from a gemmological laboratory, which will say whether the stone is natural and whether it has been treated.

What Acheson was after

Edward Acheson never did make his diamond. He made silicon carbide instead, the material of moissanite, and gave industry one of its great abrasives. Today the diamond he was chasing is grown in pressure capsules and vacuum chambers, and the crystal he found by accident sits beside it in jewellers' windows. Three stones on a tray, all sparkling: the only honest way to tell them apart is to know exactly what each one is, and to say so.