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Cubic zirconia (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). It is sometimes erroneously called “cubic zirconium”.
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 commercial production began in 1976. Its main competitor as a synthetic gemstone is a more recently cultivated material, synthetic moissanite.
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 adamantine. 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 consideredbrittle.
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.
The Soviet-perfected skull crucible is still used today, with little variation. Water-filled copper pipes provide a cup-shaped scaffold in which the zirconia feed powder is packed, the entire apparatus being wrapped with radio frequency induction coils running perpendicular to the copper pipes. A stabilizer, typically calcium oxide, is mixed with the feed powder.
The RF induction coils function in a manner similar to the primary winding in a transformer. The zirconia acts as the “secondary winding” of a transformer which in effect is “shorted” out and thus gets hot. This heating method requires the introduction of small pieces of zirconium metal. The metal is placed near the outside of the charge and is melted by the RF coils and heats the surrounding zirconia powder from the outside inwards. The cooling water-filled pipes embracing the outer surface maintain a thin “skin” (1–2 mm) of unmelted feed, creating a self-contained apparatus. After several hours the temperature is reduced in a controlled and gradual manner, resulting in the formation of flawless columnar crystals that are typically about 5 cm long by 2.5 cm wide, although they may be grown much larger. Prolonged annealing at 1400 °C is then carried out to remove any strain. The annealed crystals are then cut into gemstones.
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.
- Specific gravity: the density of cubic zirconia is about 1.7 times that of diamond.
- Refractive index: cubic zirconia has a refractive index of 2.15–2.18, compared to a diamond’s 2.42.
- 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. By comparison, 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 while diamond is one of the most efficient thermal conductors, with the thermal conductivity exceeding that of most metals. This allows diamond to be distinguished from cubic zirconia with the right instruments.