Cubic zirconia (CZ) is so optically close to diamond that only a trained eye can easily differentiate the two. There are a few key features of CZ that distinguish it from diamond, some observable only under the microscope or loupe.
For example: With a dispersive power greater than diamond (0.066 vs. 0.044) the more prismatic fire of CZ can be seen by even an untrained eye.
Hardness: CZ has an 8.5 on the Mohs Hardness Scale vs. a rating of 10 for diamonds. Meaning CZ can only be scratched by sapphires or 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. Obviously, this difference is only useful when examining loose stones.
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).
Refractive index:G> CZ has a refractive index of 2.176, compared to a diamonds 2.417.