| racing the outer surface maintain a thin "skin" (1-2 mm) of unmelted feed, creating a self-contained apparatus. After several hours the heat is reduced in a controlled and gradual manner, resulting in the formation of flawless columnar crystals. Prolonged annealing at 1400°C is then carried out to remove any strain. The annealed crystals, which are typically 5 cm long by 2.5 cm wide (although they may be grown much larger), are then cut into gemstones.
The addition of certain metal oxide dopants into the feed powder results in a variety of vibrant colors. For example:
Cerium: yellow, orange, red
Chromium: green
Neodymium: purple
Erbium: pink
Titanium: golden brown
==Innovations==
In recent years manufacturers have sought ways of distinguishing their product by supposedly "improving" cubic zirconia. Coating finished CZs in a film of diamond-like carbon (DLC) or Amorphous Diamond is one such innovation, a process using chemical vapor deposition. The resulting material is purportedly harder, more lustrous and more like diamond overall: The coating is thought to quench the excess fire of CZ, while improving its refractive index, thus bringing it more in line with diamond. Additionally, because of the high percentage of diamond bonds in the amorphous diamond coating, the finished simulant will show a positive diamond signature under Raman spectroscopy.
Another technique first applied to quartz and topaz has also been adapted to cubic zirconia: Vacuum-sputtering an extremely thin layer of metal oxide (typically gold) onto the finished stones creates an iridescent effect. This material is marketed as "mystic" by many dealers. Unlike DLC, the surreal effect is not permanent, as abrasion easily removes the oxide layer.
==CZ vs Diamond==
Cubic zirconia is so optically close to diamond that only a trained eye can easily differentiate the two. There are a few key features of CZ which distinguish it from diamond, some observable only under the microscope or loupe. For example:
Dispersion. With a dispersive power greater than diamond (0.060 vs. 0.044) the more prismatic fire of CZ can be seen by even an untrained eye.
Hardness. CZ has an 8.5 to 9.0 on the Mohs' hardness scale vs. a rating of 10 for diamonds.
Specific gravity. CZs are heavyweights in comparison to diamonds; a CZ will weigh about 1.7 times more than a diamond of equivalent size.
Flaws. Contemporary production of cubic zirconia is virtually flawless, whereas most diamonds have some sort of defect, be it a feather, included crystal, or perhaps a remnant of an original crystal face (e.g. trigons).
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