If you have a stone that you're not sure is the real thing, try these methods to find out for sure.
Step 1: Try the transparency test Flip the diamond upside down and place it on a piece of newspaper. If you can read the newspaper through the diamond, it's not a real diamond.
Step 2: Try the fog test Hold the diamond in front of your mouth and breathe on it like you would if you were trying to fog a mirror. If the stone stays fogged for two to four seconds, it's a fake. A real diamond would disperse the heat before you could get it up to your eye
TIP: The reliability of this test is suspect if there is oil and dirt on the stone, and it won't be accurate if the stone is diamond on the top with cubic zirconia glued to the bottom.
Step 3: Try the weight test Use a carat or gram scale to weigh the stone. The most common diamond fake is cubic zirconia, which weighs approximately 55 percent more than a diamond of the same shape and size.
Step 4: Try the U.V. test Hold the stone under an ultraviolet or black light. Many diamonds will display a blue fluorescence under a black light, while most fakes won't. However, a lack of blue fluorescence doesn't necessarily mean the stone is a fake. High-quality diamonds, which can be as much as 20 percent more valuable, don't show any fluorescence.
Step 5: Examine the stone under a loop Look at the stone through a loop, or jeweler's magnifying lens. A real diamond's facets will be sharp, not rolled. If the girdle is faceted or frosty, it's probably real, but if it's waxy and slick, it's probably fake.
TIP: Look for flaws in the stone, such as carbon, pinpoints, and small cracks. You'll find these kinds of things in nearly every diamond, but they're very difficult to manufacture in a fake.
Step 6: Use a diamond tester Ask a jeweler if you can use their tester, or if they will test the stone for you. Moissanite is the only stone than can fool an electronic diamond tester.
Step 7: Look at the setting stamp Look inside the setting for a stamp of "10K, 14K, 18K, 585, 750, 900, 950, PT, or Plat." If the setting is real gold or platinum, the odds are better that the stone is real. However, if there is a stamp of "C.Z.", the stone is a fake.