10 JANUARY 2008
Blue light special
Fascinated by diamonds? Then join marriageable singles, African guerillas, diamond merchants and smugglers, all of whom prize these clear carbon crystals for reasons related to size, color and value.
Still, unless it's a rock as unusual as the Hope diamond, it's often hard to tell one natural diamond from the next. The Hope, we remind you, is a high-net-worth crystal that was mined in India during the 1600s, and now lives under lock and key at the Smithsonian's National Museum of Natural History.
Not available in any stores: The Hope diamond, removed from its necklace, shows that ultra-mysterious blue allure. Don't look for this baby on eBay! Photo by Chip Clark/Smithsonian
A new study sheds light on this identification problem, and also explains why the Hope looks blood-red under ultraviolet light -- a color not always seen in other blue diamonds.
Natural diamonds are messengers from the past that formed under great heat and enormous pressure deep inside the planet, and then were rapidly lifted to the surface in a volcanic eruption.
Diamond is an ultra-hard form of crystallized carbon that refracts light -- and light is the best way to investigate diamonds, unless you are allowed to bust them up, which the Smithsonian would probably prohibit on its most popular exhibit.
So when a group of researchers from the Smithsonian, Pennsylvania State University and the Naval Research Laboratory wanted to explore the unusual glow of the Hope, they used light. More specifically, they used spectroscopy, which breaks light into its component wavelengths for analysis.
The light touch
Spectroscopy indirectly reads the activity of electrons as they orbit atoms in a sample. When electrons are exposed to energy, they can get excited and jump to a higher orbit. Then, when they drop back to their former orbit, they release energy in the form of photons -- particles of light.
Electrons on each element make a particular wavelength of light through this process, and that individuality allows spectroscopists to "read" the chemical makeup of a distant star.
The researchers lit the diamond with ultraviolet (UV) light, which is powerful enough to kick electrons to higher orbits.
The researchers could not hope to haul the Hope to a university lab, so they had to work around the museum's exhibition hours. "If you want to study the Hope diamond using spectroscopy, you need to bring the machine to the Hope diamond," says Peter Heaney, a professor of geosciences at Penn State. "You cannot bring the Hope to the machine."
Eyes on the prize
In a study of the Hope and 65 other blue diamonds, the research group found that they all glowed red, even after the UV light was turned off. Blue diamonds that did not look red to the naked eye still had that red signature through the spectroscope.
Under ultraviolet light, the Hope diamond glows red. New research explores the origin of that color. Photo by John Nels Hatelberg
The red light is photons that are emitted by electrons orbiting boron atoms trapped inside the diamond's crystal structure. Boron and any other impurities in the carbon lattice entered while the diamond crystal was forming, says Heaney. "The impurities must have been in the fluid from which the diamond crystallized. There are still some fundamental questions about how they form deep inside the Earth, what form the carbon took before it turned into diamond. So when you ask, 'Where are the impurities coming from?' 'Somehow they got incorporated' is the unsatisfying answer."
Beyond demonstrating that all natural blue diamonds glow red, a second benefit of the study comes from calculations on the spectroscopic results, which produced a unique number for each of the blue diamonds examined.
The researchers took to calling this number a "fingerprint" for the blue diamond.
Unfortunately, the fingerprint technique is unlikely to get drafted soon into the battle against conflict diamonds, Heaney says. "I had been interested in diamond science for about 15 years, trying to understand how it is that they form in the Earth," he says, "and as an outgrowth, that led to the issue of conflict diamonds," which are used to fuel many wars, especially in Africa.
A rough diamond with a sulfide inclusion from Botswana. Gem-quality diamonds lack inclusions, which makes them much tougher to identify. Photo by Jeff Harris, University of Glasgow, U.K., NSF
Conflicted over diamonds
Diamonds are highly portable, and the international ban on trading conflict diamonds would be more enforceable if diamonds could positively be traced to particular mines or countries. But the secretive diamond trade makes it hard to know if diamonds from a specific mine share a spectroscopic fingerprint. "The problem is that we don't know the geographic origin of diamonds," says Heaney, which get pooled together before sale.
Even if spectroscopic analysis does not slow the trade in conflict diamonds, it may aid jewelers, Heaney says. "An honest jeweler wants to prove to a customer that the diamond they are given to set is the same diamond they give back to the customer." Using a table-top spectrometer and other equipment, the jeweler could create a spectroscopic fingerprint, Heaney says.
Beyond their value as high-horsepower bling, Heaney says "diamonds represent the most extreme of all materials. It's the hardest material we know, has the highest thermal conductivity, very high density, and a very high brilliance. Just from a material science perspective, diamond is a remarkable solid."
Heaney adds that hanging with the Hope was an experience all its own. "Once you see it outside the setting, it is one of the most spectacular gems... My breath was taken away when I looked at it. I suddenly understood why people have marveled at it over the centuries."
- David Tenenbaum
Related Why Files
• Hope diamond at Smithsonian Institution
• Using phosphorescence as a fingerprint for the Hope and other blue diamonds, Sally Eaton-Magaña et al,
• Geology, Jan. 2008.
• Peter Heaney
Bibliography
• Canadian Diamonds
