How to Tell if a Diamond is Real

Steps

1. Ask for a certificate.grading authority (e.g. GIA, AGSL, LGP, PGGL)^[1] or an independent appraiser who is affiliated with a professional organization (like the American Society of Appraisers). This is especially important if you're buying a stone you haven't seen, such as from the Internet.
2. 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 ^[2] which is not altered in any way by a nice cut - unless, technically speaking, the cut induced a permanent strain on the crystalline lattice).
   
   
   • If the diamond is not mounted, turn it upside down and place it on a piece of newspaper. If you can read the print through the stone or even see distorted black smudges, then it probably isn't a diamond. (Unless the cut is disproportionate, in that case print can be seen through a real diamond.)^[3]
   • If the stone shows any sign of double refraction, it may be Moissanite (silicon carbide), a gemstone that is so similar to a diamond that even jewelers can have a hard time telling them apart. Look at the facet junctions from the top side of the stone, the 'star' facets' if you see what looks like double vision then that is the doubling effect.^[4]
   • If the diamond is mounted, you should not be able to see the bottom of a diamond looking directly from the top.^[5]
   • Draw a small dot with a pen on a piece of white paper. Place your unmounted diamond over the center of the dot. Look directly down on it and if your stone is not a diamond, you will see a circular reflection in the stone.
3. 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.^[6]
4. Take the bottom view: Under a microscope hold the stone table (top facet) down. If you see an orange flash only to the facets as you rock the stone it is fake.^[7]
5. Buy a combination Diamond/Moissanite Tester. These are readily available and can quickly indicate if it is a true diamond or simulant.^[8]
6. Weigh the stone. Cubic zirconia weighs approximately 55% more than diamonds for the same shape and size.^[9] Use a carat or gram scale to compare the stone in question to a real diamond.
7. Check the setting and mount. A real diamond is not likely to be set in a cheap metal.^[10] 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.^[11]
8. Put the stone under a UV light. Many (but not all) diamonds will exhibit blue fluorescence under an ultra violet or black light, so the presence of a medium to strong blue confirms that it is real. The absence of blue, however, does not mean it is fake; it could simply be a better quality diamond.^[12] If you see a very slight green, yellow, or gray fluorescence under ultraviolet light, it may be Moissanite.^[13]
9. Test it with a heat probe. Real stones disperse heat quickly and they won't heat up with the probe. This takes about 30 seconds and is often done free of charge. It also doesn't hurt the stone the way some other ways of testing will.
10. Use the fog test: Put the stone in front of your mouth and fog it like you would a mirror. If it stays fogged for a couple seconds, it’s probably a fake. A real diamond disperses the heat instantaneously so by the time you look at it, it has already cleared up, or if it happens to be a little dirty, it still clears much faster than a fake. Use a stone you know is real next to the suspect stone and fog both. You can watch how the real one stays clear while the fake one fogs over, plus if you breath on them repeatedly, you will see condensation start to build up on the fake, it fogs more with with each puff, while the real one is still clean and clear.
11. Have the diamond x-rayed. Real diamonds do not show up on an x-ray, glass, cubic zirconium and crystals all have slightly radiopaque quailities, diamonds are radiolucent.
12. Use a jeweler's loupe to inspect the diamond. Mined diamonds usually have small imperfections or inclusions that can be seen this way. Cubic zirconium does not have these imperfections. For that matter, lab-grown diamonds (which should pass all of the other tests) usually don't have imperfections either.

http://www.wikihow.com/Tell-if-a-Diamond-is-Real