The photo above shows the Hope Diamond, a 45.52-carat "blue diamond" on permanent display at the Smithsonian in Washington, D.C. When you see this King Of All Bling in regular white light, it looks blue. But if you see it under ultraviolet light, the Hope glows red. (In fact, the Hope will continue to glow—or "phosphoresce"—red even several minutes after the UV light is turned off!) Now scientists have found that this red phosphorescence may be the key to distinguishing all real blue diamonds from the fakes.
Diamonds are made of mostly carbon, and their exact color is determined by traces of other elements mixed in. Yellow diamonds, for instance, contain relatively large nitrogen impurities. Blue diamonds contain low levels of nitrogen and higher levels of boron.
Under UV light, the vast majority of blue diamonds look not red, but blue. So in the past, gemologists thought that the few blue diamonds that glowed red under UV light must have been children of the Hope (which, when originally mined from India in the 1600s, was 112 carats). But it turns out that all authentic blue diamonds contain a red phosphorescent component; it's just that for most of them, the red light is overpowered by blue-green. The researchers think that it's the precise mix of nitrogen and boron impurities that causes the lasting red glow.
The discovery was made by researchers from the Smithsonian Institution and the Naval Research Laboratory. They used a portable spectrometer to analyze several dozen blue diamonds, including the Hope, and found a red component in all but five. They also tested three phony blue diamonds—of which none had the telltale red signature. They published their findings in the January issue of the journal Geology.
Most interesting to me is that the researchers never moved the Hope from its display location. "If you want to study the Hope diamond using spectroscopy, you need to bring the machine to the Hope diamond," Penn State geoscientist Peter J. Heaney said in a press release. "You cannot bring the Hope to the machine." Heaney's team could only take measurements in the early mornings and evenings, when the museum was closed to the public.
(Flickr, via absolutwade)