Here are quick six steps to determine if your diamond is fake or real.
Certified by diamond grading authority. Make sure it’s from an impartial diamond grading authority (e.g. GIA, AGSL, EGL, PGGL) or an independent appraiser who is affiliated with a professional organization (like the American Society of Appraisers).
Look through it. Diamonds have a high "refractive index" (meaning they sharply bend the light that passes through them). Glass and quartz have a lower refractive index, meaning they sparkle less because they bend light less, even when they’ve been cut nicely (because the refractive index is an inherent physical property which is not altered in any way by a nice cut – unless, technically speaking, the cut induced a permanent strain on the crystalline lattice).
Observe the reflections. A real diamond’s reflections usually manifest in various shades of gray. If you see rainbow reflections, you’re either dealing with a low-quality diamond or a fake.
Water testing .Put the diamond in some water, if it is real it will sink, and if it is fake it will float. Put the stone in your mouth and breathe on it. If the stone stays "foggy" for 2-4 seconds, then it is not real. Real diamonds will have cleared by the time you look at them.
Weigh the stone. Cubic zirconia weighs approximately 55% more than diamonds for the same shape and size. Use a carat or gram scale to compare the stone in question to a real diamond.
Check the setting and mount. A real diamond is not likely to be set in a cheap metal. Stamps inside the setting indicating real gold or platinum (10K, 14K, 18K, 585, 750, 900, 950, PT, Plat) are a good sign, while a "C.Z." stamp will give away that the center stone is not a real diamond.