Diamond Glow

Diamonds are expensive because they're beautiful and rare. But fake diamonds often sell for a lot of money, too, because they can look very real.

Now, scientists have discovered a way to distinguish certain genuine diamonds from imitations. The simple new technique works with a rare form of blue diamond that glows in the dark.

Diamonds that belong to a group called type IIb usually look blue. After they absorb high-energy light, though, type IIb diamonds phosphoresce, or glow in the dark, for a little while. This phosphorescence ranges in color from blue to pink to fiery red, depending on the diamond.

Type IIb diamonds can be stunning, and some of them are quite famous. The large Hope Diamond, for one, glows orange-red for up to a minute after the lights go out. (The Hope Diamond is on display at the Smithsonian Institution's National Museum of Natural History in Washington, D.C.)

Despite these diamonds' rarity and fame, however, scientists hadn't paid much attention to them till recently.

To learn more about the stones, chemical engineer Sally Eaton-Magaña of the Gemological Institute of America in Carlsbad, Calif., and her colleagues studied a set of diamonds called the Aurora Heart Collection. The set contains 239 colored diamonds, including many blue, type IIb gems.

They also studied the Smithsonian's Hope Diamond and its Blue Heart Diamond. In all, the researchers did experiments with 67 natural blue diamonds, three manmade gems, and a gray diamond that scientists had turned blue with a combination of temperature and pressure treatments.

In one test, the scientists shone ultraviolet light—a type of high-energy light—on each gemstone for 20 seconds. Afterward, all the natural type IIb diamonds glowed for several seconds.

Measurements revealed that this glow contained two wavelengths of visible light: greenish-blue and reddish. The relative strength of each wavelength determined the color of the final glow. And because each diamond is different, the scientists could use the color of the glow and how quickly the glow fades as a sort of fingerprint to identify individual gems.

The technique also proved to be a good way to separate the real gems from the fakes. Neither the manmade diamonds nor the falsely colored gray diamond glowed in the reddish wavelength. The new strategy might help solve one of the diamond market's biggest problems: hard-to-spot fakes.