Cubic Zirconia (CZ) is a synthetic gemstone that does not occur in nature in a form suitable for jewelry. Since its commercial introduction in the 1970s, it has become the most popular and affordable diamond simulant on the market. The material’s brilliance and clarity are carefully engineered through a sophisticated industrial process.
The Chemical Composition of Cubic Zirconia
The core substance of cubic zirconia is Zirconium Dioxide, represented by the chemical formula ZrO2. While this compound exists naturally, the synthetic gemstone requires the material to be in the cubic crystalline form of ZrO2, which is crucial for its diamond-like optical properties.
At normal temperatures, Zirconium Dioxide naturally forms a monoclinic structure, which is not suitable for a gemstone. To stabilize the material into its isometric, or cubic, crystal system, a small percentage of a stabilizing agent is introduced during synthesis. This stabilizer is typically yttrium oxide (Y2O3) or calcium oxide (CaO), which prevents the crystal from reverting to its natural structure upon cooling. The name “cubic zirconia” directly reflects this stabilized arrangement of zirconium and oxygen atoms.
Manufacturing Cubic Zirconia Using Skull Melting
The creation of usable cubic zirconia crystals relies on a specialized technique known as skull melting. This method is necessary because Zirconium Dioxide has an extremely high melting point, reaching approximately 2,750°C (4,982°F). No conventional crucible material, such as platinum, can withstand this heat without melting or contaminating the batch.
The skull melting process overcomes this challenge by allowing the material to form its own container, or “skull.” Raw ZrO2 powder, mixed with a stabilizer, is placed in a water-cooled metal container with a radio-frequency (RF) induction coil wrapped around it. The RF coil generates intense heat, melting the core of the powder while the outer layer, cooled by the surrounding water, remains solid.
This solidified outer layer acts as a self-contained, high-purity crucible for the molten ZrO2 core. A small piece of zirconium metal is often added initially to help the non-conductive powder begin absorbing the RF energy. Once the entire inner mass is molten, the RF energy is slowly reduced, initiating a controlled cooling process that allows large, single columnar crystals to grow from the bottom up.
Key Physical Differences from Diamond
Although cubic zirconia is an excellent diamond simulant, its chemical composition results in measurable physical differences.
Hardness
One significant variation is in hardness, where CZ scores between 8 and 8.5 on the Mohs scale, while diamond scores 10. This lower hardness means that the facets of CZ are more susceptible to abrasion and scratching over time, which can dull the stone’s surface.
Density
Cubic zirconia is considerably denser than diamond. CZ has a specific gravity ranging from 5.6 to 6.0 g/cm3, compared to diamond’s 3.52 g/cm3. As a result, a CZ stone will weigh approximately 1.6 to 1.7 times more than a diamond of the exact same size and cut.
Optical Properties
The optical properties also differ, particularly in the measure of light dispersion, which causes the “fire” or colored flashes seen in a gemstone. CZ has a much higher dispersion rate (0.058–0.066) than diamond (0.044). This means it separates white light into spectral colors more dramatically. This excessive “rainbow sparkle” can sometimes be a telltale sign that the stone is a cubic zirconia.