There are several ways to tell if a diamond is real, but the easiest is to take it to a licensed jeweler. If this is not an option, there are several home-test methods to determine a diamond's legitimacy, such as pen marking or weighing the gem.
If determined to test the diamond without a jeweler's help, whether the diamond is loose or mounted, usually determines how to test it. The pen mark method only works for loose diamonds, but is the easiest. The diamond should be placed on a white piece of paper with a small mark or dot, with the flat side of the diamond place directly onto the dot. In a fake diamond, the dot will reflect through as a ring, but a real diamond breaks up the image of the dot hundreds of times, causing the stone to darken, but no dot or ring is visible.
For weight testing, it is important to know the carat weight of the diamond and the weight of the mounting. A jeweler's scale can be used to weigh the gemstone and subtract the mounting weight from the total. A real diamond will weigh about 200 milligrams per carat while a cubic zirconium will be almost twice that number.