How can you tell whether a diamond is real?

Answer 1

Ask a Jeweler

Assuming the diamond is yours*, the traditional way is to try to cut glass with it, although this is not recommended, as it may damage the setting. The easiest way is simply to walk into any jewelry store that buys jewelry (many do) and tell them you're thinking of selling it. They will pull out a jeweler's loup, examine the stone and size it in about two minutes. If the stone is a fake, they'll tell you instantly. Just be aware that if the stone is genuine, the price they quote may be about half what you originally paid for the new ring.

Pay for a Certificate

Take your diamond to a certified gemologist and pay for a certificate that will document the diamond's weight, cut, clarity and colour.

Use a thermal probe

Many jewelry stores and almost all pawn shops have a thermal probe to test diamonds to determine if they are real. The probes are generally a bit expensive (a few hundred $US), and are difficult to fool.

Glass Test Doesn't Cut it

Tests like 'scratching glass' are very old tests derived from when many imitation diamonds were in fact made from glass. Most modern diamond simulants including CZ, YAG, or moisanite can readily cut glass so this test no longer provides any useful information.

Real or Man-Made

Identification of synthetic diamond (man-made diamond) from real diamond is almost impossible with hand-held tools such as thermal probes. If you have to know, you can get your stone tested by a reputable gemological lab.

* If you're speculating about someone else's ring, it's difficult to impossible to be sure. Cubic Zirconias and other fakes look so convincing that people couldn't tell if Paris Hilton's megawatt stone was real or not. I'd try to judge by the person's general socioeconomic status. Someone who buys their clothes at Walmart is unlikely to be sporting a genuine five-carat stone. Celebrities and the wealthy DO wear fakes from time to time, especially to prevent heavy loss if theft occurs when traveling.
Test your stone with thermal probe.

Answer 2

There are a number of different ways to tell. The quickest and easier is the fog test: a real diamond will disperse the heat from your breath for immediately, the setting: such as 10k, 14k or 18k stamps inside the setting, anything with a C.Z (cubic zirconia) usually indicates that the diamond is not real, a jewelers loupe and also if it has rainbow reflections if you look at it from the top.