Jan. 8, 2008 -- The eerie smoldering glow of the world's largest rare blue diamond has finally been explained -- and it's neither unique nor a mere oddity.
The Hope Diamond is a 45.52-carat, deep-blue diamond that sits center stage in the Smithsonian Institution's United States Gem Collection. The object of intrigue for almost four centuries, the diamond has more recently been puzzling scientists because of the deep red glow it gives off for several seconds after being bathed in ultraviolet light.
"It was thought to be quite rare," said Naval Research Laboratory gem researcher Sally Magaña. "It added to its intrigue and mystery."
Magaña led a team of researchers studying the phosphorescence of the Hope Diamond and 66 other rare natural blue diamonds -- including the world's second-largest Blue Heart blue diamond. The diamonds were available briefly at the same location from the Aurora Heart and Aurora Butterfly diamond collections.
They used the latest digital spectral measurement devices to measure the wavelengths of light emitted by the diamonds, along with the rate the glowing faded. Their results appear in the January issue of the journal Geology.
The researchers discovered that, in fact, all natural blue diamonds phosphoresce with red or blue-green light in various individual, quirky ways. Some, like the Hope Diamond, give off far less green-blue light, which allows their red glow to dominate. The cause of the phosphorescence is primarily the element boron, although the finer details of the visual effect are still not worked out, she said.
"It's really not that rare," Magaña told Discovery News, of the Hope Diamond's glow.
Nor is the Hope Diamond the best at the glow-in-the-dark trick, she said, "It's not the longest lasting or the most intense."
Other blue diamonds glow red, she said, and some glow much longer -- up to 28 seconds in one case, compared to the Hope Diamond's 8.2-second glow.
That said, there's no denying that the size of the Hope Diamond makes the natural special effect more impressive.
"Because it's so large it looks like a fiery coal in your hand," said Magaña.
What may be more important, however, is that the unique mix of colors given off by different individual diamonds, combined with each diamond's individual rate of fading, provide a new way to fingerprint any natural blue diamond, explained Magaña, who recently moved to the Gemological Institute of America.
That means some long-standing questions about whether certain smaller stones were really cut from larger stones can be sorted out once and for all. They all should share the same phosphorescent signature.
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