Ways to tell if your diamond is real or fake.
1) Never look at a loose diamond on a white jewelers'tray. Always look at it on a piece of newspaper or printed material. If the diamond is real, you won't be able to read the newsprint through the facets; if the diamond is fake, it's just like looking through, well, glass.
2) One of the best test is thermal conductivity. Diamonds are extremely good conductors of heat and most jewelers have a small instrument that can measure this. No other material will match a diamond in this test.
3) The fog test. This test I like a lot. Put the rock in front of your mouth and fog it like you would try to fog a mirror. If it stays fogged for two to four seconds, it’s a fake. A real diamond disperses the heat instantaneously so by the time you look at it, it has already cleared up. (Sometimes oil and dirt on the stone can effect its reliability and the test is not accurate at all on doublets where the top of the stone is diamond and the bottom is cubic zirconia epoxied together.)
4) Under the loop test. If you own some sort of magnifying lens, there are some things you can look for on the stone that might give away its identity:
A. Look at the rock from the top and see how well the facets (cuts on top of the diamond) are joined. They should be sharp not rolled.
B. Look at the girdle and see if it is faceted or frosty (a clear sign it’s a diamond) or waxy and slick (an indication it’s a fake).
C. While you’re looking at your stone under magnification, look into your stone to see if you detect any flaws (carbon, pinpoints, small cracks). These are typically clear indications it’s the real thing since it’s very hard to put inclusions in a fake.
D. After examining the stone, focus in on the stamps inside the setting. A stamp of "10K, 14K, 18K, 585, 750, 900, 950, PT, Plat" indicates the setting is real gold or platinum which gives a better chance that the stone in it is real as well. While you’are looking at the interior of the ring, also look for any "C.Z." stamps that would indicate the center stone is not a diamond.