To tell the difference between diamond and cubic zirconia, breathe on the gem to fog it up. If it clears up instantly, there is a good chance it is a real diamond because diamonds are good at conducting heat. Alternatively, find a real diamond of roughly equal size to compare the weight of the two objects. Cubic zirconia is about 50 percent heavier than diamond.
Unlike cubic zirconia, most real diamonds have internal flaws. Use a jeweler's loupe to inspect the gem in good lighting, and look for small imperfections that resemble tiny feathers. Cubic zirconia is internally flawless and does not contain such imperfections.
Real diamonds pass the scratch test. Take the gem and try to scratch glass. If the gem does not scratch glass, it is likely cubic zirconia. However, since other synthetic diamonds scratch glass, passing this test does not guarantee that the gem is real.
For larger gems, conduct the newspaper test by placing the gem on top of a piece of newspaper with fine print. A real diamond refracts the light and makes the print illegible, while the print is visible and clear under cubic zirconia.
While at-home tests help determine the likelihood that a gem is cubic zirconia or diamond, use pen-size probes called "diamond testers" to test the thermal conductivity to be certain. Alternatively, consult a professional jeweler or gemologist.