The famed Hope Diamond glows a mysterious red when exposed to ultraviolet light, a finding that scientists say can help them "fingerprint" blue diamonds and tell the real ones from the artificial.
The phosphorescence comes from boron in the gem, the same element that makes it appear blue in normal light, explained Jeffrey Port, curator of the National Gem Collection at the Smithsonian's National Museum of Natural History.
But while all blue diamonds glow in ultraviolet light, most glow blue. The Hope glows red, indicating a different mix of boron and nitrogen, Post explained in a telephone interview.
He said researchers, by measuring the different glows, have been able to tell real blue diamonds from artificial ones as well as real ones that have been "enhanced" in laboratories.
The research was done at the Smithsonian and Naval Research Laboratory and their findings are reported in the journal Geology.
Some historians believe the Hope Diamond was cut from a larger gem first found in India and later part of the French crown jewels before the French Revolution.
If that is the case, Post said, the tests could also be used to identify other stones from the same source.
The 45.52-carat blue Hope Diamond is on display at the Natural History museum, but Post said lighting conditions there don't allow it to be shown in ultraviolet light. He said the museum hopes to make a video of the stone when it glows — which continues for some time after the light is turned off — so visitors can see that.
"People typically think of the Hope Diamond as a historic gem, but this study underscores its importance as a rare scientific specimen that can provide vital insights into our knowledge of diamonds and how they are formed in the earth," said Post.
Prior to this study, only limited scientific research existed regarding the phosphorescence properties of natural blue diamonds. Due to the rarity and extreme value of blue diamonds, scientists had typically used synthetic diamonds in past research. Post and his colleagues' recent research took advantage of a unique opportunity to examine a large collection of natural blue diamonds at the museum that were made available by diamond dealers.