Cubic zirconia (or CZ), is the cubic crystalline form of zirconium dioxide (ZrO2). 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 competitor for diamonds since 1976. Its main competition as a synthetic gemstone is the more recently cultivated material, synthetic moissanite.

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.[citation needed] During synthesis zirconium oxide would otherwise form monoclinic crystals, its stable form under normal atmospheric conditions. The stabilizer is required for cubic crystal formation; for example CZ rings and cubic zirconia earrings 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 bracelets and CZ pendants has no cleavage and exhibits a conchoidal fracture. It is considered brittle. It is considered brittle because although it has a hardness of 8.5 it can still easily be fractured because the hardness of a mineral refers to its tendency of being scratched.

Under shortwave UV cubic zirconia necklaces and dog tags 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.

In recent years manufacturers have sought ways of distinguishing their product by supposedly "improving" cubic zirconia anklets. Coating finished CZs in a film of diamond-like carbon (DLC) or Amorphous Diamond is one such innovation, a process using chemical vapor 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, while improving its refractive index, thus bringing it more in line with diamond. Additionally, because of the high percentage of diamond bonds in the amorphous diamond coating, the finished simulant will show a positive diamond signature under Raman spectroscopy.

Another technique first applied to quartz and topaz has also been adapted to bangles / cuffs: 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.

There are a few key features of cubic zirconia silver sets which distinguish it from diamond, some observable only under the microscope or loupe, for example:[citation needed]

* Dispersion: With a dispersive power greater than diamond (0.060 vs. 0.044) the more prismatic fire of cubic zirconia charms can be seen by even an untrained eye.

* Hardness: Cubic zirconia has an 8.5 to <9.0 on the Mohs hardness scale vs. a rating of 10 for diamonds.

* Specific gravity: Cubic zirconia crystals are heavyweights in comparison to diamonds; a cubic zirconia will weigh about 1.7 times more than a diamond of equivalent size.

* Flaws: Contemporary production of cubic zirconia is virtually flawless,[citation needed] 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).

* Refractive index: CZ cufflinks has a refractive index of 2.176, compared to a diamond's 2.417.

* Cut: Under close inspection with a loupe, the facet shapes of some cubic zirconias appear different from diamonds.

* Color (or more precisely, the lack thereof): Only the rarest of diamonds are truly colorless, most having a tinge of yellow or brown to some extent. By comparison, cubic zirconia can be made in most cases entirely colorless: equivalent to a perfect "D" on diamond's color grading scale.

* Thermal conductivity: Cubic zirconias are thermal insulators while diamonds are among the most efficient thermal conductors, exceeding copper. This makes telling the difference between diamond and cubic zirconia quite easy for those with the right instruments.