Cubic Zirconia (or CZ) is zirconium oxide (ZrO2), a mineral that is extremely rare in nature, but is widely synthesized for use as a diamond simulant. The synthesized material is hard, optically flawless and usually colourless, but may be made in a variety of different colours. It should not be confused with zircon, a zirconium silicate (ZrSiO4).

Because of its low cost, durability, and close visual likeness to diamond, synthetic cubic zirconia has remained the most gemologically and economically important diamond simulant since 1976. Its main competition as a synthetic gemstone is the more recently cultivated material moissanite.

Technical aspects
Cubic zirconia is, as its name would imply, crystallographically isometric: As diamond is also isometric, this is an important attribute of a would-be diamond simulant. Synthesized material contains a certain mole percentage (10-15%) of metal oxide stabilizer. During synthesis zirconium oxide would otherwise form monoclinic crystals, as that is its stable form under normal atmospheric conditions. The stabilizer is required for cubic crystal formation; it may be typically either yttrium or calcium oxide, the amount and stabilizer used depending on the many recipes of individual manufacturers. Therefore the physical and optical properties of synthesized CZ vary, all values being ranges.

It is a dense substance, with a specific gravity of between 5.6-6.0. Cubic zirconia is relatively hard, at about 8.5 on Mohs scale - nowhere near diamond, but much harder than most natural gems. Its refractive index is high at 2.15-2.18 (B-G interval) and its lustre is subadamantine. Its dispersion is very high at 0.058-0.066, exceeding that of diamond (0.044). Cubic zirconia has no cleavage and exhibits a conchoidal fracture. It is considered brittle.

Under shortwave UV cubic zirconia typically luminesces a yellow, greenish yellow or "beige." Under longwave UV the effect is greatly diminished, with sometimes a whitish glow being seen. Coloured stones may show a strong, complex rare earth absorption spectrum.

History
Since 1892 the yellowish, monoclinic mineral baddeleyite had been the only natural form of zirconium oxide known. Being of rare occurrence it had little economic importance.

The extremely high melting point of zirconia (2750°C) posed a hurdle to controlled single-crystal growth, as no existing crucible could hold it in its molten state. However, stabilization of zirconium oxide had been realised early on, with the synthetic product stabilized zirconia introduced in 1930. Although cubic, it was in the form of a polycrystalline ceramic: it was made use of as a refractory material, highly resistant to chemical and thermal (up to 2540°C) attack.

Seven years later, German mineralogists M. V. Stackelberg and K. Chudoba discovered naturally occurring cubic zirconia in the form of microscopic grains included in metamict zircon. Thought to be a byproduct of the metamictization process, the two scientists did not think the mineral important enough to formally name. The discovery was confirmed through x-ray diffraction, proving a natural counterpart to the synthetic product exists.

As with the majority of diamond imitations, the conceptual birth of single-crystal cubic zirconia began in the minds of scientists seeking a new and versatile material for use in lasers and other optical applications. Its evolution would eclipse earlier synthetics, such as synthetic strontium titanate, synthetic rutile, YAG (Yttrium Aluminium Garnet) and GGG (Gadolinium Gallium Garnet).

Some of the earliest research into controlled single-crystal growth of cubic zirconia occurred in 1960s France, much work being done by Y. Roulin and R. Collongues. The technique developed saw molten zirconia contained within itself with crystal growth from the melt: The process was named cold crucible, an allusion to the system of water cooling used. Though promising, these pursuits yielded only small crystals.

Later, Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow perfected the technique, which was then named skull crucible (an allusion either to the shape of the water-cooled container or to the occasional form of crystals grown). They named the jewel Fianit, but the name was not used outside of the USSR. Their breakthrough was published in 1973 and commercial production began in 1976. By 1980 annual global production had reached 50 million carats (10,000 kg).

Synthesis
The Soviet-perfected skull crucible is still used today, with little variation. Water-filled copper pipes provide a cup-shaped scaffold in which the zirconia feed powder is packed, the whole contraption being wrapped with radio frequency induction coils running perpendicular to the copper pipes. A stabilizer is mixed with the feed powder, being typically either yttria or calcium oxide.

The RF induction coils function in a manner similar to a microwave. This heating method requires the introduction of a solid piece of zirconium metal as a catalyst: The metal is melted by the RF coils and heats the surrounding zirconia powder from the centre outwards. The cooling water-filled pipes embracing the outer surface maintain a thin "skin" (1 mm) of unmelted feed, creating a self-contained apparatus. After several hours the heat is reduced in a controlled and gradual manner, resulting in the formation of flawless columnar crystals. Prolonged annealing at c. 1400°C is then carried out to remove any strain. The annealed crystals, which are typically 5 cm long by 2.5 cm wide (although they may be grown much larger), are then cut into gemstones.

The addition of certain metal oxide dopants into the feed powder results in a variety of vibrant colours. For example:

Cerium: yellow, orange, red
Chromium: green
Neodymium: purple
Erbium: pink
Titanium: golden brown

Innovations
In recent years manufacturers have sought ways of distinguishing their product by supposedly "improving" cubic zirconia. Coating finished CZs in a film of diamond-like carbon (DLC) is one such innovation, a process using vapour deposition. The resulting material is purportedly harder, more lustrous and more like diamond overall: The coating is thought to quench the excess fire of CZ, bringing it in line with diamond.

Another technique first applied to quartz and topaz has also been adapted to cubic zirconia: Vacuum-sputtering an extremely thin layer of metal oxide (typically gold) onto the finished stones creates an iridescent effect. This material is marketed as "Mystic" by many dealers. Unlike DLC, the surreal effect is not permanent, as abrasion easily removes the oxide layer.

CZ versus diamond
Cubic zirconia is so ostensibly close to diamond that only a trained eye can easily differentiate the two. There are a few key features of CZ which clearly distinguish it from diamond, some observable only under the microscope or loupe. For example:

Dispersion. The superlative fire of CZ makes for a somewhat gaudy display, enough for the trained eye to gauge.
Hardness. The inferior hardness of CZ (8.5 vs. 10 of diamond) manifests itself in the gem's lower lustre, rounded facet edges and surface scratches.

Specific gravity. CZs are heavyweights in comparison to diamonds; a CZ will weigh about 1.7 times more than a diamond of equivalent size. Obviously, this difference is only useful when examining loose stones.

Flaws. Contemporary production of cubic zirconia is virtually flawless: most diamonds have some sort of birthmark, be it a feather, included crystal, or perhaps a remnant of an original crystal face (e.g. trigons).

Cut. CZs are generally cut and polished to lesser standards than diamond; although not apparent to the eye, a loupe is enough to reveal the sloppily arranged facets of most CZs.

Colour. More precisely, the lack of colour: Only the rarest of diamonds are truly colourless, most having a tinge of yellow or brown to some extent. By comparison, CZ can be made entirely colourless: equivalent to a perfect "D" on diamond's colour grading scale. Furthermore, the fancy colours of CZ in no way approximate the shades of fancy diamonds.

Thermal conductivity. This is probably the most important property of diamond from a jeweller's perspective: all she need do is apply the tip of a thermal probe to a suspect diamond. CZs are thermal insulators whilst diamonds are among the most efficient thermal conductors, exceeding copper.

Further reading
Gems Made by Man by Kurt Nassau, 1980. (ISBN 0-8733-016-1)