Synthesis
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 "skin" (1–2 mm) 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 that are typically about 5 cm long by 2.5 cm wide, although they may be grown much larger. Prolonged annealing at 1400°C is then carried out to remove any strain. The annealed crystals are then cut into gemstones.
The addition of certain metal oxide dopants into the feed powder results in a variety of colors. For example:
| Dopant | Color(s) |
|---|---|
| Cerium | yellow |
| orange | |
| red | |
| Chromium | green |
| Neodymium | purple |
| Erbium | pink |
| Titanium | golden brown |
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Purple cubic zirconia with checkerboard cut
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Multi-colour cubic zirconia
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Three-tone cubic zirconia gems
Read more about this topic: Cubic Zirconia
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