Some research is best done behind heavy, steel-clad doors. For example, when Smithsonian geologist Jeffrey Post embarked on an experiment involving the Hope Diamond, at 45.52 carats, the world’s largest deep-blue diamond and one of the most famous and valuable museum objects on earth, he and his colleagues locked themselves and the diamond inside a large vault in the depths of the Smithsonian’s National Museum of Natural History.

Post and fellow diamond investigators from the U.S. Naval Research Laboratory, Ocean Optics Co. and Pennsylvania State University were investigating a phenomenon that the millions of museum visitors who gaze at the diamond on its rotating pedestal behind bulletproof glass never see—the mysterious red phosphorescent glow the stone emits when exposed to ultraviolet light. In the end, Post not only learned more about the Hope Diamond; he discovered a sophisticated technique for confirming the identity of any blue diamond.

The Hope Diamond’s red glow has long been considered a unique property of that stone. Most blue-colored diamonds produce a bluish-white phosphorescence if exposed to ultraviolet light, and the few other diamonds known to emit red phosphorescence were commonly assumed to have been from the even larger original stone from which the Hope was cut. “It was something that always intrigued people,” says Post, who is curator of the National Gem and Mineral Collection. “For people who like the whole idea of a curse and that kind of story, the fact that this thing phosphoresces a bloody red color is just too good to be true.”

One owner to another

Much that is known about the Hope Diamond seems too good to be true. Formed deep within the earth more than a billion years ago and brought to the surface by a volcanic eruption in India, the diamond was discovered in the mid-1600s. Sold in 1668 to King Louis XIV of France, it was stolen during the French Revolution, only to reappear decades later in London. Passing from one owner to another, the diamond along the way acquired a reputation for bearing a curse that brought misfortune to its possessors.

In 1911, the legendary jeweler Pierre Cartier sold the deep-blue stone to Evalyn Walsh McLean, a Washington, D.C., socialite who sometimes fastened the diamond to her dog’s collar. The gem changed hands again in 1949, when it was bought by New York diamond merchant Harry Winston. Fifty years ago, on Nov. 10, 1958, Winston donated the diamond to the Smithsonian. Winston seems to have had more faith in the

U.S. Postal Service than in the curse, electing to mail the stone to Washington in a plain brown package.

Mindful that many museum visitors want to see the Hope Diamond when in Washington, Jeffrey Post timed his research to occur in the hours after the museum closed for the evening and before it reopened the next day. With guards standing by, the diamond was removed from its pedestal in the Harry Winston Gallery.

“We had a local jeweler come in and take it out of its setting for us,” Post says. (The jeweler returned early the following morning to reset the stone.) Next, Post and his colleagues locked themselves in the Department of Mineral Sciences’ room-sized vault with the diamond and a portable high-sensitivity spectrometer.

Phosphorescent spectrum

“The clock was running,” Post says, and the scientists got to work. The diamond was positioned on a piece of clay inside a box that could be sealed to keep out ambient light. A fiber-optic cable connected to an ultraviolet light source was extended into the box and “pushed up against the top face of the Hope Diamond,” Post says. Then the diamond was exposed to ultraviolet light for several seconds.

When the light source was turned off, the diamond began to emit its characteristic red glow, a phenomenon that lasts several minutes. A second fiber-optic cable in the box channeled phosphorescent light from the diamond to the spectrometer.

“That was the first time we had been able to see a display of what the phosphorescence spectrum looked like for the Hope Diamond,” Post says. Spectrometers measure wavelengths of light, and the display on a laptop computer hooked to the spectrometer revealed that the Hope Diamond’s red light was more than just red.

“We saw two strong peaks in the spectrum,” Post explains. One was in the red portion of the spectrum, “but the second peak, interestingly, was in the green portion of the spectrum.” Because they were able to measure the spectrum approximately every second throughout the period of phosphorescence, Post and his colleagues saw that the green peak decayed quickly. “After the first few seconds, it’s gone,” Post says. The red peak, however, lasted much longer. That slow decay in the red portion of the spectrum accounts for the diamond’s overall red glow.

Diamond district
In the vault that night, the scientists also collected spectrum readings from other diamonds, including the second-largest known deep-blue diamond, the 30.62 carat Blue Heart, also in the Smithsonian’s gem collection. The phosphorescence from several synthetic diamonds was also measured. Later, Post and his colleagues brought their portable spectrometer to New York City’s Diamond District, an area of Manhattan that is a center of the world’s diamond industry, where a dealer known to Post granted the researchers access to dozens of valuable blue diamonds in his safe for further spectral measurements.

The scientists have learned that all blue diamonds show red and green peaks in their phosphorescence spectrum. But the relative intensity of those peaks and the rate at which they decay varies from diamond to diamond, leading to differences in the phosphorescent glow seen by the naked eye. The synthetic diamonds they measured showed a completely different range of spectrum readings, with no red peaks.

Boron-nitrogen interaction

Diamonds are composed of carbon, and impurities in the carbon give rise to a stone’s color. Blue diamonds have relatively high levels of boron impurities but low levels of nitrogen. Post believes that the red phosphorescence emitted to some degree by all blue diamonds is likely due to interaction between those two elements. To test that possibility, he is continuing his study of blue diamonds, using a different kind of instrument that lets him measure the amount of boron and nitrogen in individual stones “and then correlate that with the particular spectra that we’re getting off those diamonds,” he explains.

Post’s work has already yielded knowledge of interest not just to scientists but to diamond sellers and their customers. Because the relative intensity of the blue and red components of each blue diamond’s phosphorescence is unique, the same sort of analysis that the researchers did in the museum vault might be used to fingerprint individual blue diamonds and to distinguish natural stones from man-made ones.

Since even a half-carat natural blue diamond can sell for tens of thousands of dollars, dealers and customers have an interest in knowing all they can about the pedigree of a stone. “Are you really buying a natural blue diamond or not—that kind of information becomes very valuable on the marketplace,” Post says. “Likewise, if a certain blue diamond disappears and suddenly three cut blue diamonds appear on the market, you might want to know, ‘Did those three cut diamonds come from that stolen diamond?’”

No stranger to spectacular gems, Post still gets a thrill from working with the Hope Diamond. “Every time I look at it, I kind of go, ‘My gosh!,’” he says, noting that it is impossible to overlook the stone’s “human history, the curse and the kings, the queens, the thefts.” However, the scientist believes that his recent research underscores the famous diamond’s importance as “a unique natural history object.”

“It’s a good reminder of why we keep objects like this in museums,” Post says. “We keep them here so that we have access to them, so that we can continue to study them and learn from them. I fully believe that 10 years from now, or a 100 years from now, there will be different spectrometers. There will be different questions. There will be people with new curiosities. There will be some other reason why we want to learn something even more fundamental about this very unusual and very important diamond.”

And the only sort of places where that is going to be possible, he says, “are going to be places like the Smithsonian. Because, where else?”