What cubic zirconia is

The mineral nobody named

In 1937, two German mineralogists, M. von Stackelberg and K. Chudoba, were studying a badly weathered zircon sent to them by the British gemmologist B. W. Anderson. Inside it they found a few tiny crystals and, using X-rays, identified them as a cubic form of zirconium oxide.

They thought so little of the find that they didn't give the new mineral a name. That is why, nearly ninety years on, we still call it by its plain scientific description: cubic zirconia. It took another four decades, and a laboratory in Moscow, to turn those specks into the stone now set in rings and earrings all over the world.

What it is made of

Cubic zirconia, or CZ, is zirconium dioxide, ZrO₂, held in a cubic crystal structure. That last part takes some doing. Left to itself, zirconia changes its crystal structure as it heats and cools. It only becomes cubic at very high temperatures, around 2,000°C, and it changes back as it cools.

The trick is to add a stabiliser. A small amount of yttrium oxide (yttria) or calcium oxide (lime) mixed into the zirconia stops it changing back, so the cubic form survives all the way down to room temperature. Yttria is the usual choice in gem material.

A word on names. "Cubic zirconium" and plain "zirconium" are both wrong: zirconium is a metal. The stone is zirconia, the oxide.

How it is made: skull melting

Pure zirconia melts at about 2,750°C. No crucible can hold it at that temperature without melting or contaminating it. The solution is wonderfully simple: let the zirconia hold itself.

1. A cup of copper fingers. Zirconia powder, mixed with its stabiliser, is packed into a cup made from a ring of water-cooled copper tubes with small gaps between them.
2. Radio-frequency heating. A coil around the cup sends radio-frequency energy through the gaps. Cold zirconia powder doesn't conduct electricity, so a few pieces of zirconium metal are added to start the heating.
3. The skull. The powder melts from the inside out, except for a thin shell next to the cold copper, which stays solid. The molten zirconia ends up sealed inside a crust of its own powder. That crust is the "skull" that gives the method its name.
4. Slow cooling. The melt is held for some hours, then lowered slowly out of the coil, typically over about twelve hours. Crystals form at the bottom and grow upwards in parallel columns until the whole melt has set.
5. Breaking out. Light tapping separates the columns. They are then cut and polished like any other gem.

From laser research to jewellery

Nobody set out to make a diamond imitation. Scientists wanted large zirconia crystals for lasers and electronics, and the gem came as a happy side effect.

It arrived under a crowd of trade names. A 1981 review in GIA's journal Gems & Gemology lists more than twenty, from Djevalite and Fianite to Diamonique III. Diamonique is still a CZ brand today.

CZ and diamond compared

Cubic zirconia is not a diamond and is not made of carbon. It is a different material that happens to look a lot like one. These are GIA's figures for both.

In short, GIA describes CZ as having slightly more fire but less brilliance than diamond. The extra fire shows as flashes of rainbow colour; the lower brilliance means a little less white sparkle. The figures for CZ are given as ranges because its exact recipe varies between makers.

The weight difference has a practical side. Because CZ is so dense, a CZ of a given size is heavier than the same size of diamond. That is why CZ is often sold by its size in millimetres rather than by carat weight.

How it is used

Most CZ is colourless and made to look like diamond, and most of it has high clarity with few, if any, flaws. It suits anywhere a bright, clear sparkle is wanted at a modest price: studs, solitaire rings, pavé bands and tennis bracelets.

It can also be made in almost any colour. Small amounts of other elements are added to the melt: cerium for yellows, oranges and reds, erbium for pinks, and so on. Coloured CZ became popular from 1979, when cutters at the Tucson Gem and Mineral Show bought up every piece of coloured rough on offer. Its high refractive index gives coloured CZ more life than many natural coloured stones.

Outside jewellery, stabilised zirconia was in use long before the gem, as a ceramic for very high temperatures, because it has an exceptionally high melting point and resists most chemicals.

Telling CZ from diamond

A good CZ can fool the eye. GIA is clear that sparkle proves nothing: almost any faceted gem will sparkle to an untrained eye, and the only certain way to identify a stone is in a gemmological laboratory. Some general points still help.

• Weight. In a loose stone, CZ's high density is easy to notice: it feels heavier than a diamond of the same size.
• Heat testers. Diamond conducts heat exceptionally well, and a jeweller's thermal tester separates it from CZ. But synthetic moissanite conducts heat so well that thermal testers read it as diamond, so a jeweller needs more than one test.
• Fire. CZ's stronger dispersion can show as more rainbow colour than a diamond of similar cut. It is a clue, not a proof.
• Ignore the myths. GIA says the fog test can't be relied on and the read-through test should never be used as a definitive test. The scratch test is destructive and should never be used.

CZ is not zircon

The names are close, and the confusion is old. GIA describes zircon as a gem "unfairly confused with cubic zirconia". They are entirely different things.

Zircon is one of the most remarkable minerals on earth. Crystals found in Australia are 4.4 billion years old, the oldest known. It is one of the December birthstones, and blue is its most popular colour, usually produced by heating. Colourless zircon was widely used to imitate diamond in the early 1900s, long before cubic zirconia replaced it in that role.

There is one family connection: the natural cubic zirconia found in 1937 was discovered inside a zircon. That is as close as the two get.

Caring for CZ

CZ is hard enough for everyday jewellery, but it is not as hard as diamond or sapphire, and GIA notes it can be somewhat brittle. GIA also notes that colourless CZ can yellow over time. A few simple habits help.

• Clean it gently. Warm water, a couple of drops of washing-up liquid and a soft brush, such as a soft toothbrush. Rinse well and dry with a lint-free cloth.
• Store pieces apart. Keep jewellery in a soft-lined box or separate pouches so stones and metals don't rub and scratch one another.
• Take it off for rough jobs. Remove rings for anything that could knock or scratch them, and keep jewellery away from chemicals, especially chlorine.
• Check the setting. The metal around the stone needs care too. Silver, plated finishes and gold each have their own needs.

A name that stuck

Von Stackelberg and Chudoba never imagined what would become of the specks they found in 1937. They didn't even think them worth naming. Today their nameless mineral, grown by the tonne in skulls of its own powder, sparkles in jewellery everywhere, still answering to the plain description they gave it.