Cubic zirconia is a synthetic gemstone, but although manufacturers create it in a lab, it is a distinct substance from lab-created gemstones. Jewelers use the term "lab-created" to describe gems that are identical to natural gems in every way but their synthesis. Cubic zirconia has its own set of chemical and physical properties that distinguish it from any other gem, natural or lab-created.
Jewelers market the crystalline form of zirconium oxide as cubic zirconia. Manufacturers add other elements to color the finished product, but all cubic zirconia is some form of zirconium oxide and has the general chemical formula ZrO2. Other gems, whether natural or lab-created, have their own unique chemical formulas. Diamonds, for example, consist of carbon. Rubies and sapphires, forms of the mineral corundum, also bear the name aluminum oxide, as these are their primary ingredients. Emeralds, aquamarines and morganites share the beryl family's combination of beryllium, aluminum and silicate (SiO3) molecules.
As all gems have their own chemical composition, all of them have their own unique specific weight. Zirconium is a heavier element than the carbon, beryllium and aluminum that make up the bulk of the precious gemstones; cubic zirconia is likewise heavier than any of the precious gems. A cubic zirconia that shares the exact dimensions as a similar diamond will weigh about 1.55 times more than the diamond. Lab-created gems have the same chemical structure as natural gems and therefore share their specific weight.
Cubic zirconia has a hardness of 8.5 on the Mohs hardness scale, placing it between topaz at 8 and the corundum gemstones at 9. Diamonds are the toughest gem, with a 10 on the Mohs scale of hardness. Although natural and lab-created gems have the same hardness, inclusions and fractures can compromise a natural gem's intrinsic hardness. Lab-created gems represent the mineral's ideal hardness, and three of the four precious gems are significantly harder than cubic zirconia; hardness testing can differentiate lab-created gems from cubic zirconia. To distinguish cubic zirconia from softer gems, jewelers examine the stone under a microscope. Softer gems such as iolite, tanzanite and citrine show tiny nicks and abrasions on their facet edges, while a harder cubic zirconia gem will not.
Gemologists and jewelers refer to a gem's ability to scatter white light into spectral colors as its dispersion. A well-cut clear diamond shows strong dispersion, or fire, as light plays across it, but a cubic zirconium displays even more fire. Diamonds have a dispersion rate of 0.44, while cubic zirconia has a dispersion of 0.66. Both natural and lab-created emeralds, rubies and sapphires exhibit low dispersion and range from 0.14 to 0.18. A gemstone with the rich colors of an emerald or ruby but the fire of a diamond is almost certainly a cubic zirconia.