How to Test a Diamond

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Blue diamond ring
A blue diamond that fetched 6.2 million pounds at auction on April 24, 2013. (Image credit: Bonhams)

In the past, one of the most common techniques to test if a diamond was real or synthetic was to scratch the gem against glass — if the glass is scraped or scratched, the diamond's real.

But this method is not foolproof, as some faux diamonds can also scratch glass. So how can you tell if a diamond is real?

One technique involves placing the gem over a newspaper or other printed text — real diamonds refract light so much that you wouldn't be able to see any lines, circles or full letters coming through. But a diamond that's cut too shallow may produce inconclusive results.

Another test requires breathing on the gem like you would a mirror. Diamonds are good at conducting heat, so the fog your breath creates should clear up almost instantly. However, a synthetic rock called moissanite can pass this test.

Because different stones have different densities, you can also compare how heavy your gem is to a real diamond of roughly equal size. For instance, if your stone is a cubic zirconia, it will be about 50 percent heavier than a real diamond of equal size.

If you really want to be positive, you need to go electronic. Commercial pen-size probes, called simply "diamond testers," can test the thermal conductivity of your gem.

However, moissanite registers as a diamond with these devices, just like with the breath test. So, you may need another probe that reads the stone's electrical conductivity (or a dual tester that measures both properties).

Of course, these tests don't tell you anything about the value of your diamond. According to the Gemological Institute of America, jewelers grade diamonds on the "4Cs" — cut, clarity, color and carat weight.

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Joseph Castro
Joseph Castro
Live Science Contributor
Joseph Bennington-Castro is a Hawaii-based contributing writer for Live Science and Space.com. He holds a master's degree in science journalism from New York University, and a bachelor's degree in physics from the University of Hawaii. His work covers all areas of science, from the quirky mating behaviors of different animals, to the drug and alcohol habits of ancient cultures, to new advances in solar cell technology. On a more personal note, Joseph has had a near-obsession with video games for as long as he can remember, and is probably playing a game at this very moment.