Naturally occurring zirconium oxide (zirconia) is extremely rare. However, a unique laboratory process called skull melting can be used to grow very large crystals of zirconia; these have a cubic internal structure, and cubic zirconia is widely known as CZ. It can be made in a wide variety of colors (as in the photo) and cut gems were found to have brilliancy and dispersion (play of colors) strikingly similar to diamond. CZ is now extensively used as a diamond simulant.