Hope Diamond's Red Glow Explained

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.