Cubic zirconia, also known as CZ, is the crystalline form of the mineral zirconium oxide. It is a hard and optically perfect material. Even though it can be made in various colors, it is usually colorless. Naturally occurring cubic zirconia was discovered by German mineralogists M. V. Stackelberg and K. Chudoba in 1937.
At the time, it was not thought of to have any economic or other useful importance. In the 1960s, researchers Y. Roulin and R. Collongues attempted to manufacture cubic zirconia using the cold crucible process, which only resulted in producing small crystals. Eventually, Soviet scientists in 1970s discovered a breakthrough that allowed cubic zirconia to be grown in the lab. Their technique was called the skull crucible.
The skull method is a process in which microwave technology is used to melt the cubic zirconia into a molten state in which it creates its own crucible. Essentially, the material inside heats up and the outside remains cool resulting in a harden shell or “skull”. After several hours of the heating process, the material is left to cool and harden under strictly controlled conditions. The material must undergo a sustained heat of a temperature of 1400°C to remove any strain.
As a result, crystals are formed while it hardens, which are usually 5 cm long by 2.5 cm wide, sometimes larger. These crystals are then cut to produce the completed gemstone. Since cubic zirconia crystals are grown in the lab, the finished gemstones usually are perfect and can be made any desired color. The skull crucible process is still used today despite several attempts to improve the process.