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Why does the Hope diamond glow red?
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he Hope diamond is famous for its size and notorious for bringing misfortune to its owners, but the real mystery is why this blue diamond glows with a fiery, reddish-orange phosphorescence under UV light. The diamond is the biggest attraction at the Smithsonian’s Natural History Museum and has drawn hundreds of millions of people during the 50 years it’s been on exhibit. “It became one of the collection icons,” says Jeffrey Post, the curator-in-charge of the mineral collection at the Smithsonian. “If you look at the attendance figures for the museum prior to the Hope diamond coming and after, [museum attendance] shot up dramatically after the diamond came here.” But few in the general public have witnessed the stone’s stunning color change. “It looks like you are holding a glowing coal, like from a barbeque grill or something, in your hand,” says Post. “It’s not a subtle phenomenon.” Post and colleagues wanted to find out what caused this rare and brilliant glow. “Frankly, it’s always been kind of embarrassing to not be able to explain to people why it does that,” says Post. “Part of the problem with really studying it, though, is that it is not a trivial thing to take the Hope diamond to a laboratory.” Fortunately, a modern, portable visible-light spectrometer recently allowed the laboratory to come to the diamond. In a new study, Post and colleagues dissected the phosphorescence of the gem down to its individual wavelengths and compared the Hope diamond’s phosphorescence with that of other, less famous blue diamonds (Geology 2008, 36, 83–86).
The Hope diamond mystique
Part of the fascination with the Hope diamond lies in its long and torrid history. The original rough diamond— © 2008 American Chemical Societ y
(a)
Chip Clark/Smithsonian and John Nels Hatelberg
Despite old rumors, chemistry—not a curse—is the key.
(b)
The Hope diamond (a) under ambient light and (b) showing its “bloody” red phosphorescence after exposure to UV light.
probably the largest dark-blue diamond ever mined—was found in India in the 1600s. A French diamond merchant sold it to King Louis XIV of France, who had it cut down to a 67 carat stone known as the French Blue diamond. After more than a century as part of the French crown jewels, the stone was stolen during the French Revolution in 1792. The French Blue has never been seen again. About 20 years later, a slightly smaller blue diamond appeared for sale in London. Even then, many suspected that this new diamond was the French Blue, cut to disguise its origins. Because blue diamonds are very rare, their movements tend to be well documented. “It’s hard to lose a blue diamond in history, because if one big, blue diamond disappears and is never seen again, and then suddenly, ~20 years later, another big, blue diamond appears, you start to wonder,” Post says. That stone eventually came into possession of the eponymous Hope family. After passing through the hands of several other owners, the diamond was donated to the Smithsonian in 1958. The public fascination with the Hope diamond did not begin with its museum debut, however. Evelyn Walsh McLean, a wealthy socialite who owned the gem during the early part of the
20th century, wore it everywhere. It was during her ownership that reports of a “curse” on the diamond started to surface. Despite her wealth, McLean suffered several tragic events during her life, which proponents of the curse theory blame on the diamond. That’s when public interest in the stone took off. “It’s the combination of this big, blue diamond [with] this interesting history, and then you throw in a curse or two and suddenly it becomes a bigger-thanlife gemstone,” Post says.
Moving past the myth
Why does the stone have this unusual phosphorescent glow? Some believe in a supernatural explanation. “People who like the curse story love thinking that, ‘Gee, this phosphorescence’—they like to refer to it as the bloody red phosphorescence—‘clearly is connected to the whole curse thing,’” Post says. “But there have been reports in the literature of other blue diamonds phosphorescing.” Some of these diamonds glow red, some white, and some green, and the scientists wanted to find out what caused these differences. Ocean Optics, a company that makes portable visible-light spectrometers, provided an instrument to the researchers, who set it up in a secure vault and started taking time-resolved phospho-
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rescence spectra of the Hope and other blue diamonds in the Smithsonian’s collection. What they found was actually quite surprising: even though the phosphorescent colors looked very different to the naked eye, all of the blue diamonds’ spectra contained the same peaks: one in the green-blue region (~500 nm) and one in the red region (~660 nm) of the spectrum. The relative size and decay rates of these peaks are what determined the color of each diamond’s visible phosphorescence. The Hope diamond, for example, has an intense red peak that decays very slowly and a weaker green peak that decays quickly. The Smithsonian’s other large blue diamond, the Blue Heart diamond, has the same two peaks in its spectrum. But in this case, the green peak dominates, and both peaks decay fairly rapidly, leading to a white, shortlived phosphorescent glow. To get more information on the very early stages of the diamonds’ phosphorescence, the researchers used time-resolved photoluminescence spectroscopy, which can measure a spectrum in 250 colored diamonds. This collection contained many smaller blue diamonds, which enabled researchers to broaden the scope of their study. They found that each diamond had a unique “fingerprint” when it came to the green/red peak intensity ratio and decay time of the red peak. Post says that these data could ultimately help gemologists pinpoint diamonds that are cut from the same 2296
Few in the general public have witnessed the Hope diamond glow like a fiery red coal under UV light. original stone or identify gems that are stolen and subsequently resurface with a different cut. “There’s a certain forensic aspect of it that could be useful,” he says. “The fact that these spectrometers are relatively inexpensive and very portable means that it’s not difficult to measure these spectra, and that makes it something that might be applicable to the gemology world.” The other application of this technique might be in identifying synthetic or lab-enhanced stones. Blue diamonds can be grown under high-pressure and high-temperature conditions in the presence of boron, which imparts the blue color. Or, natural white diamonds can be treated with boron to add a blue tint. “More and more synthetic diamonds are being offered,” says Post. “Once they get out on the market, sometimes they start to get mixed [in] a little bit” with natural stones. The researchers included three boron-doped synthetic blue diamonds in their study and found that their phosphorescence spectra were very different from the natural stones. None of the synthetic diamonds displayed the 660 nm peak, but they did have the peak at 500 nm. Some had an additional peak at ~575 nm, and all the synthetic diamonds had much higher emission intensities than the natural stones. Although the sample size in this case was relatively small, Post says that this technique looks promising as another tool to identify synthetic diamonds.
The source of the glow
Scientists have known for many years that the blue color of the diamonds is caused by defects in the crystal lattice where boron has taken the place of carbon. Could boron also cause the phosphorescence?
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Because the intensity and decay rate of the green and red peaks depend on temperature, the scientists measured the diamonds’ phosphorescence at 35–155 °C. The researchers modeled this data using the Arrhenius equation and calculated the activation energy of the phosphorescence to be ~0.4 eV for the 660 nm peak. The researchers postulate that the phosphorescence is caused by donor–acceptor pair recombination (DAPR). Essentially, the donor provides electrons to the acceptor, and the pair emits light with a distinct energy. When boron acts as an acceptor, its binding energy is 0.37 eV—very close to the activation energy calculated from the temperature-dependence experiments. “It made sense that these features that we’re seeing in the phosphorescence spectrum in fact were also arising from the boron, the very impurity that was giving the [blue] color,” says Post. But the scientists are still missing one piece of the puzzle: if boron is the acceptor, what is acting as the donor? Post says they strongly suspect that small amounts of nitrogen—possibly in two different kinds of defects, which would explain the two different observed peaks—are acting as the donor in this DAPR system. “At this point, we think that the differences that we are seeing are due to the relative amounts of boron and nitrogen in the diamonds,” says Post. He adds that they want to use TOF secondary ion MS to look for small amounts of nitrogen in the diamonds and then correlate their spectroscopy data with the variations in boron/nitrogen. Post says that work on diamonds is ongoing. “Now that we have the spectrometer here, anytime we see anything interesting we go ahead and measure a spectrum,” he says. “The difference is that in the past it was always a more visual description, whereas now, by looking at the spectra in some detail, we can actually get a better sense of what’s going on from a structural point of view.” To the long and varied history of the Hope diamond—the theft, the intrigue, and the curse—Post and colleagues have added a new chapter: science. a —Jennifer Griffiths
