Cubic Zirconia differs from most gems in that it has been entirely synthesised. The man-made cubic zirconia, is often completely colourless, or equivalent to a perfect D on the diamond scale, although it can be manufactured in colours of great variety and intensity. It is extremely hard and durable, making an excellent substitute for diamonds, which is chiefly its intended purpose.
Advances in modern technology have allowed for greater advances in ensuring that the match to diamonds is as close as possible. A microscope is needed to tell the difference. Recently the Cubic Zirconia has been covered with a layer of carbon at the end of the manufacturing process. This has helped to make it harder, although still not to the same degree as a diamond, and achieved a more radiant, diamond-like look overall.
Cubic Zirconia can be made with metal oxides to create any colour desired, or no colour at all. It is a popular alternative to more expensive gemstones in costume jewellery to help keep costs down, especially when creating pieces which would call for multiple gemstones to create the desired look.