Cubic zirconia (or CZ), the cubic crystalline form of zirconium dioxide (ZrO₂), is a mineral that 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, synthetic moissanite.

Technical aspects

As its name would imply, cubic zirconia is 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 (7-20%) of metal oxide stabilizer.Fact: date=November 2007 During synthesis zirconium oxide would otherwise form monoclinic crystals, 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— 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 a whitish glow sometimes being seen. Colored stones may show a strong, complex rare earth absorption spectrum.

History

Discovered in 1892, the yellowish monoclinic mineral baddeleyite is a natural form of zirconium oxide. It has little economic importance because of its rarity.

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.