Jewelry: Cubic Zirconia

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 colorless, but may be made in a variety of different colors. It should not be confused with zircon, which is 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, and 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 between 5.6 - 6.0. Cubic zirconia is relatively hard, at about 8.5 on the 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 luster 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. Colored 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 realized 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).

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia Cubic Zirconia.

History of Jewelry: Cubic Zirconia