Cubic zirconia (CZ) is the cubic crystalline form of zirconium dioxide (ZrO<sub>2</sub>). The synthesized material is hard 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 (ZrSiO<sub>4</sub>). 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.

Technical aspects

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 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 density between 5.6 and 6.0&nbsp;g/cm<sup>3</sup>—about 1.65 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 lustre. 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.

History

Discovered in 1892, the yellowish monoclinic mineral baddeleyite is a natural form of zirconium oxide. Their breakthrough was published in 1973, and commercial production began in 1976. 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.

Synthesis

thumb|Worker monitoring melting zirconium oxide and yttrium oxide in an induction-heated "cold crucible" to create cubic zirconia

The skull-melting method refined by Josep F. Wenckus and coworkers in 1997 remains the industry standard. This is largely due to the process allowing for temperatures of over 3000&nbsp;°C to be achieved, lack of contact between crucible and material as well as the freedom to choose any gas atmosphere. Primary downsides to this method include the inability to predict the size of the crystals produced and it is impossible to control the crystallization process through temperature changes.

!Symbol

!Color(s)

|-

|Cerium

|Ce

| yellow-orange-red

|-

|Chromium

|Cr

|green

|-

|Cobalt

|Co

|lilac-violet-blue

|-

|Copper

|Cu

|yellow-aqua

|-

|Erbium

|Er

|pink

|-

|Europium

|Eu

|pink

|-

|Iron

|Fe

|yellow

|-

|Holmium

|Ho

|Champagne

|-

|Manganese

|Mn

|brown-violet

|-

|Neodymium

|Nd

| purple

|-

|Nickel

|Ni

|yellow-brown

|-

|Praseodymium

|Pr

|amber

|-

|Thulium

|Tm

|yellow-brown

|-

|Titanium

|Ti

|golden brown

|-

|Vanadium

|V

|green

|}

{| class="wikitable"

|+

!Color Range This material is marketed as "mystic" by many dealers. Unlike diamond-like carbon and other hard synthetic ceramic coatings, the iridescent effect made with precious metal coatings is not durable, due to their extremely low hardness and poor abrasion wear properties, compared to the remarkably durable cubic zirconia substrate.

Comparison with diamond

Key features of cubic zirconia distinguish it from diamond:

thumb|alt=A triangular facet of a crystal having triangular etch pits with the largest having a base length of about 0.2 mm|One face of an uncut octahedral diamond, showing trigons (of positive and negative relief) formed by natural [[Chemical milling|chemical etching]]

  • Hardness: cubic zirconia has a rating of approximately 8 on Mohs hardness scale vs. a rating of 10 for diamond.

Regarding value, the paradigm that diamonds are costly due to their rarity and visual beauty has been replaced by an artificial rarity The company pleaded guilty to these charges in an Ohio court on 13 July 2004. However, the KP is not as effective in decreasing the number of conflict diamonds reaching the European and American markets. Its definition does not include forced labor conditions or human rights violations. A 2015 study from the Enough Project, showed that groups in the Central African Republic have reaped between US$3&nbsp;million and US$6&nbsp;million annually from conflict diamonds. UN reports show that more than US$24&nbsp;million in conflict diamonds have been smuggled since the establishment of the KP. Diamond simulants have become an alternative to boycott the funding of unethical practices. However, concerns from mining countries such as the Democratic Republic of Congo are that a boycott in purchases of diamonds would only worsen their economy. According to the Ministry of Mines in Congo, 10% of its population relies on the income from diamonds.

Further reading

fr:Zircone