Cubic Zirconia (CZ)

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.

CZ versus Diamond

Cubic zirconia is so optically 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. With a dispersive power greater than diamond (0.060 vs. 0.044) the more prismatic fire of CZ can be considered excessive and is a relatively obvious give away to even an untrained eye.

• Hardness. The inferior hardness of CZ (8.5 vs. 10 of diamond) manifests itself in the gem's lower luster, 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. Whereas most diamonds have some sort of defect, be it a feather, included crystal, or perhaps a remnant of an original crystal face (e.g. trigons).

• Index of refraction. CZ has a lower index of refraction than diamond.
  • This allows more light to leak out of a CZ, especially when greasy or wet.
  • CZ's lower index of refraction causes it to have less luster than diamond.

• Cut. Under close inspection with a loupe, the facet shapes of some CZs appear different from diamonds.
  • In theory, many gems (such as CZs and diamonds) look best when the star facet, crown main facets, and upper girdle facets do not quite meet. (Per Step 11 of editor's note 36 to Marcel Tolkowsky's Diamond Design.) Diamond has such a high refractive index that having these facets meet at a single point does not cause much loss of fire or reflection. Diamonds normally have these facets meet at a point, because that is more symmetrical and reflects well on the cutter's precision. On the other hand, CZ has a considerably lower refractive index than diamond. CZs are often cut with 6-sided crown main facets, so that the star facets do not touch the upper girdle facets. This optimizes the brilliance and fire of the CZs.
  • The optimum angle of the main crown facets is steeper for diamond than for CZ. (According to Tolkowsky's model of the crown, for a given pavillion angle and girdle thickness). CZs are often cut so that the crown main facets do not touch the girdle. This allows the CZs to have a shallower crown angle, while still having the same crown height as the diamonds being simulated.

• Color. More precisely, the lack of color: Only the rarest of diamonds are truly colorless, most having a tinge of yellow or brown to some extent. By comparison, CZ can be made entirely colorless: equivalent to a perfect "D" on diamond's color grading scale. Furthermore, the fancy colors 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 they 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.

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