Cubic zirconia (CZ) is the cubic crystalline form of zirconium dioxide (ZrO2). The synthesized material is hard, optically flawless and usually colorless, but can be manufactured in many different colors. It should not be confused with zircon. Zircon is a zirconium silicate (ZrSiO4). It is sometimes mistakenly named cubic zirconium.
Because of its cheap price, durability, and visual similarity to diamond, synthetic cubic zirconia is the most important competitor for diamonds since 1976.
Cubic zirconia is crystallographically isometric, an important attribute of a diamond simulant. Zirconium oxide would naturally form mono-clinic crystals during synthesis, its stable form under normal atmospheric conditions. A stabilizer is required for cubic crystals to form, and remain stable at normal temperatures. This may be typically either yttrium or calcium oxide, the amount of stabilizer used depending on the many recipes of individual manufacturers. Therefore all properties of synthesized CZ are different.
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 very hard, at about 8 on the Mohs scale, 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, bigger than that of diamond (0.044). Cubic zirconia because of its high hardness, it is generally considered brittle. Under shortwave UV cubic zirconia usually fluoresces a yellow, or beige. Under long wave UV the effect is greatly diminished, with a whitish glow sometimes being seen. Colored stones may have a strong, complex rare earth absorption spectrum.
History of cubic zirconia
Zirconium oxide was discovered in 1892, the yellowish mono-clinic mineral baddeleyite is a natural form of it. It is economically not too important because it is very rare.
Zirconia has a very high melting point (2750°C / 4976°F). It makes the controlled growth of single crystals difficult, as no existing crucible could hold the material in its molten state. However, stabilization of cubic zirconium oxide had been realized early on, with the synthetic product stabilized zirconia introduced in 1930.
As with the majority of grown diamond substitutes, the idea of producing single-crystal cubic zirconia arose in the minds of scientists seeking a new and versatile material for use in lasers and other optical applications. Its production eventually exceeded that of earlier synthetics.
Some of the earliest research into controlled single-crystal growth of cubic zirconia occurred in 1960s France. Molten zirconia being contained within a thin shell of still-solid zirconia, with crystal growth from the melt. The name of the process was cold crucible. This was an allusion to the system of water cooling used. Though promising, these attempts yielded only small crystals.
Later, Soviet scientists in Moscow invented a more perfect technique, which was called skull crucible. By 1980 annual total production had reached 50 million carats (10 tonnes).
According to the latest technology worker monitoring melting zirconium oxide and yttrium oxide in an induction heated cold crucible to create cubic zirconia.
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 whole contraption 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, 1–2 mm skin 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. Prolonged annealing at 1400°C is then carried out to remove any strain. The annealed crystals, which are typically 5 cm long by 2.5 cm wide, but they also may be grown much larger, are then cut into gemstones.
The addition of certain metal oxide dopants into the feed powder results a wide scale of colors. For example: Cerium=yellow, orange, red; Chromium=green; Neodymium=purple; Erbium=pink; Titanium=golden brown.