Cubic zirconia (CZ) is classified as a diamond simulant – a stone that looks similar to natural diamonds, but is made of different material. Natural and lab-grown diamonds are made of carbon, while cubic zirconia is made of zirconium dioxide (ZrO2). The cubic part of the name comes from the fact that the stone has a cubic crystalline form. Diamonds also have a cubic crystalline form, so cubic zirconia is a great diamond lookalike.