• Science & Nature

Glow-in-the-Dark Jewels

How the Hope Diamond's mysterious phosphorescence led to "fingerprinting" blue diamonds

• By Kenneth R. Fletcher
• Smithsonian.com, January 14, 2008

Observing the afterglow of the world’s largest deep-blue diamond has produced a unique identification method that could help track stolen gems or pick out phony diamonds from natural stones.

The new study was triggered by a curious habit of the Smithsonian’s 45.5 carat Hope Diamond, possibly the most-viewed museum piece in the world.

The Hope has long been known to emanate an eerie reddish-orange glow for a few minutes after being exposed to ultraviolet light, but the phosphorescence was poorly understood, says Jeffrey Post, the curator of the National Gem and Mineral Collection at Smithsonian’s National Museum of Natural History and one of the researchers of the study.

To study the phenomenon, Post and other scientists went into the museum’s vault after hours with a portable spectrometer, a machine that can measure the intensity and duration of phosphorescence.

While the glow was thought to be unique to just a few blue diamonds, the researchers discovered that almost all emit a glow after exposure to ultraviolet radiation. The report in the January edition of the journal Geology suggests that measuring the glow can lead to a unique “fingerprint” in blue diamonds that could aid in exposing diamond fraud.

Blue diamonds get their color from traces of boron. They are some of the most rare and valuable diamonds in the world, making up only one out of several hundred thousand diamonds, Post says.

The glow is believed to be an interaction between ultraviolet light, boron and nitrogen in the stones. While most blue diamonds appear to glow bluish-green after ultraviolet exposure, the study showed that blue often covers up a red phosphorescence. The Hope simply has a stronger red glow than most.

When the ratio between blue and green was first plotted, along with the duration of the glow, researchers could not find a pattern.

“We were struck by how much data scattered,” Post says. “Then it dawned on us that the fact that the data does scatter so well is a good thing, because it means that each of these diamonds has its own unique behavior, or its own fingerprint.”

He believes the relative amounts of boron and nitrogen could cause the variations in phosphorescence among natural blue diamonds.

Scientists also observed a markedly different glow in synthetic and altered diamonds.

The most immediate application of blue diamond fingerprinting could be distinguishing phony diamonds from the real thing, says Peter Heaney, professor of geosciences at Penn State University who also worked on the study.

Because fake diamonds are increasingly realistic, when you bring a valuable stone to the jeweler to have work done “you want to be sure that the blue diamond you are getting back is the same one you brought into a jeweler,” he says.

Post says that the method “could be very helpful” in tracking stolen diamonds by matching the diamond’s fingerprint with a suspected recut version.

The best news is that the fingerprinting method is non-invasive and will not damage the stone, Heaney says, which allowed the researchers to work with 67 valuable natural blue diamonds and three synthetic ones in the Smithsonian and private collections.

But Heaney says that because of the rarity of blue diamonds, especially those with known origins, it is uncertain whether the technology could be used in other applications, like identifying where a diamond came from. Knowing origins could help reduce the sale of conflict diamonds, whose trade fuels wars in parts of Africa.

Still, Post says the easy-to use, portable and relatively inexpensive spectrometer could be another tool for “checking and making sure that a particular stone has all the right characteristics of being a natural stone.”

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