Cubic zirconia is crystallographically isometric, an important attribute of a would-be diamond simulant. During synthesis zirconium oxide would naturally form monoclinic crystals, its stable form under normal atmospheric conditions. A stabilizer is required for cubic crystals to form, and remain stable at ordinary temperatures; this may be typically either yttrium or calcium oxide, the amount of 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 and 6.0 — at least 1.6 times that of diamond. Cubic zirconia is relatively hard, 8–8.5 on the Mohs scale— slightly harder than most semi-precious natural gems. Its refractive index is high at 2.15–2.18 (compared to 2.42 for diamonds) and its luster is vitreous. 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. Because of its high hardness, it is generally considered brittle.
Under shortwave UV cubic zirconia typically fluoresces 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.Cubi
Because of cubic zirconia's isomorphic capacity it can be doped with several elements to change the color of the crystal. A list of specific dopants and colors produced by their addition can be seen below.
Yellow cubic zirconia
Cubic zirconia versus diamond
There are a few key features of cubic zirconia which distinguish it from diamond:
- Hardness: cubic zirconia has a rating of approximately 8 on Mohs hardness scale vs. a rating of 10 for diamond. This causes sharp edges in cut crystals to dull and round off in CZ, while with diamond the edges remain sharp. Furthermore, when polished, diamond will rarely show polish marks and those seen will travel in different directions on adjoining facets while CZ will show polishing marks along the same direction of the polish.
- Specific gravity (relative density): the density of cubic zirconia is about 1.7 times that of diamond. This difference allows skilled gem identifiers to tell the difference between the two by weight. This property can also be exploited by dropping the stones in heavy liquids and compare their relative sink times (diamond will sink more slowly than CZ).
- Refractive index: cubic zirconia has a refractive index of 2.15–2.18, compared to a diamond's 2.42. This led to the development of immersion techniques of identification. In this method, stones with refractive indices higher than that of the liquid used will have dark borders around the girdle and light facet edges while those with indices lower than the liquid will have light borders around the girdle and dark facet junctions.
- Dispersion is very high at 0.058–0.066, exceeding a diamond's 0.044.
- Cut: cubic zirconia gemstones may be cut differently from diamonds. The facet edges can be rounded or "smooth".
- Color: only the rarest of diamonds are truly colorless, most having a tinge of yellow or brown to some extent. A cubic zirconia is often entirely colorless: equivalent to a perfect "D" on diamond's color grading scale. Other desirable colors of cubic zirconia can be produced including near colorless, yellow, pink, purple, green, and even multicolored.
- Thermal conductivity: Cubic zirconia is a thermal insulator whereas diamond is the most powerful thermal conductor. This provided the basis for Wenckus’ identification method (currently the most successful identification method)