Glow-in-the-Dark Jewels

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

Jeweler Harry Winston donated the famous Hope Diamond—the largest-known deep blue diamond in the world—to the Smithsonian Institution in 1958. It arrived in a plain brown package by registered mail, insured for one million dollars. Surrounded by 16 white pear-shaped and cushion-cut diamonds and hanging from a chain with 45 diamonds, the rare gem attracts 6 million visitors a year to the Natural History Museum. (Chip Clark)

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.


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