Cubic zirconia is crystallographically isometric, an important attribute of a would-be diamond simulant. During synthesis zirconium oxide naturally forms monoclinic crystals, which are stable form under normal atmospheric conditions. A stabilizer is required for cubic crystals (taking on the fluorite structure) to form, and remain stable at ordinary temperatures; typically this is 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.
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, such as synthetic strontium titanate, synthetic rutile, YAG (yttrium aluminium garnet) and GGG (gadolinium gallium garnet).
Some of the earliest research into controlled single-crystal growth of cubic zirconia occurred in 1960s France, much work being done by Y. Roulin and R. Collongues. This technique involved molten zirconia being contained within a thin shell of still-solid zirconia, with crystal growth from the melt. The process was named cold crucible, an allusion to the system of water cooling used. Though promising, these attempts yielded only small crystals.
Later, Soviet scientists under V. V. Osiko at the Lebedev Physical Institute in Moscow perfected the technique, which was then named skull crucible (an allusion either to the shape of the water-cooled container or to the form of crystals sometimes grown). They named the jewel Fianit after the institute’s name FIAN (Physical Institute of the Academy of Science), but the name was not used outside of the USSR. Their breakthrough was published in 1973, and commercial production began in 1976. In 1977 cubic zirconia began to be mass-produced in the jewelry marketplace by the Ceres Corporation with crystals stabilized with 94% yttria. Other major producers include Taiwan Crystal Company Ltd, Swarovski and ICT inc. By 1980 annual global production had reached 60 million carats (12 tonnes) and continued to increase with production reaching around 400 tonnes per year in 1998.
all cubic zirconia used in jewelry has been synthesized, or created by humans.
Due to its optical properties YCZ (yttrium cubic zirconia) has been used for windows, lenses, prisms, filters and laser elements. Particularly in the chemical industry it is used as window material for the monitoring of corrosive liquids due to its chemical stability and mechanical toughness. YCZ has also been used as a substrate for semiconductor and superconductor films in similar industries.
Mechanical properties of partially stabilized zirconia (high hardness and shock resistance, low friction coefficient, high chemical and thermal resistance as well as high wear and tear resistance) allow it to be used as a very particular building material. Particularly in the bio-engineering industry, it has been used to make reliable super-sharp medical scalpels for doctors that are compatible with bio tissues and contain an edge much smoother than one made of steel.
There are a few key features of cubic zirconia which distinguish it from diamond:One face of an uncut octahedral diamond, showing trigons (of positive and negative relief) formed by natural chemical etching
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 comparing 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 has led to the development of immersion techniques for identification. In these methods, 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.
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 provides the basis for Wenckus’ identification method (currently the most successful identification method
Manufacturers have sought ways of distinguishing their product by supposedly “improving” cubic zirconia. Coating finished cubic zirconia with a film of diamond-like carbon (DLC) 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 cubic zirconia, while improving its refractive index, thus making it appear more like diamond. Additionally, because of the high percentage of diamond bonds in the amorphous diamond coating, the finished simulant will show a positive diamond signature in Raman spectra.
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.
Dopant | Color(s)
Cerium – yellow-orange-red
Chromium – green
Cobalt – lilac-violet-blue
Copper – yellow-aqua
Erbium – pink
Europium – pink
Iron – yellow
Holmium – Champagne
Manganese – brown-violet
Neodymium – purple
Nickel – yellow-brown
Praseodymium – amber
Thulium – yellow-brown
Titanium – golden brown
Vanadium – green
