This process was perfected in USSR specifically for crystallizing synthetic cubic zirconia, for uses in optical, electronic and laser equipment. Cubic zirconium oxide has a very high melting point and is a very reactive material. No container can hold this melt since cubic zirconia has a melting point of 2750°C, and hence a cold crucible or skull is used.
How gemstones are synthesized by skull melting process?
- A container is filled with powdered zirconia, stabilizer and some pieces of zirconium metal.
- These are rapidly heated by a radio frequency field (similar to a microwave oven).
- The zirconium oxide powder is used as its own insulation and container as the temperature at the core is raised to extreme levels.
- The metal reacts with the oxygen and melts, with the exception of a skin of about 1/15 inches thick at the outside, which is kept solid by contact with the liquid – cooled copper tubes around – the skull.
- Zirconia and the stabilizer contents are kept molten for several hours to provide uniformity.
- The power is reduced and the skull is slowly lowered out of the coil.
- Crystal growth begins at the bottom of the skull and columnar crystals grow upwards until the entire melt solidifies.
- The individual single crystal columns that are readily available are generally 2 inches in diameter.
- Light tapping will separate the crystal columns.
- Dopants are used to produce various colours.
- Colourless when pure (without any dopants) – diamond simulant.
- The S.G. ranges from 5.60 to 6.20 (as per dopants used)
- Isotropic character.
- The dispersion is 0.060
- Some varieties exhibit a strong yellow fluorescence under ultra violet light.
- The characteristic inclusions seen are negative crystals, streams of zirconium oxide powder which appear as tiny white spherical bubbles.