The Hope Diamond is one of the most-viewed museum pieces in the world, but visitors never witness the mysterious red glow it emits when exposed to ultraviolet light. Although researchers have long known about the fiery phosphorescence of the famed blue gem that was once owned by King Louis XV, they were able to study the phenomenon in greater detail thanks to a newly developed portable spectrometer that could be brought into the diamond's vault. The device measured the wavelengths of light emitted by the Hope and 66 other blue diamonds in the Smithsonian and private collections. Scientists were surprised to discover that each produced a unique glow with variations in intensity, color and duration—making it possible to "fingerprint" each stone. Jeffrey Post, curator of the National Gem and Mineral Collection at the Natural History Museum, says that with the growing sophistication of synthetic diamonds, the method might be a "quick and easy way" to pick out fakes and could even help track stolen gems.