January 8, 2008

Hope Diamond’s phosphorescence key to fingerprinting

Teams of researchers from Penn State, the Smithsonian Institution, and the Naval Research Academy have recently discovered a trait of blue diamonds that could prove to be invaluable. When a white or natural light shines on the Hope Diamond, it glows a brilliant blue; when an ultraviolet light shines on the Hope Diamond, it glows a bright red-orange for five minutes.

Most diamond colors fluoresce; they only emit visible light as long as they are stimulated with uv radiation. Teams of researchers found that the Hope Diamond shows phosphorescence, that is it emits light after it is no longer lit. This discovery can help to distinguish synthetic and altered diamonds from real ones and may provide a way to fingerprint blue diamonds for the purpose of identification.

When this red-orange glow was first seen, researchers hypothesized that blue diamonds that glow red must have come from the parent of the Hope Diamond. That parent diamond was a 112 carat blue diamond that was cut down to 67 carats for French kings, got lost in the revolution and reappeared in 1812 as a 45 carat stone, the Hope Diamond. The Hope Diamond has a history of being one of the most sought-after museum objects in the world, but the Smithsonian is not interested in popularity, it sees its pieces as objects of scientific value and spends time, energy and resources on research.

A portable spectrometer was brought to the Hope Diamond for testing, and tests were run on a variety of other blue diamonds including the Blue Heart, the Portuguese Diamond and 64 other blue diamonds. The testing procedure is noninvasive; it simply measures the wavelengths of light emitted by an object. The researchers studied the diamonds' reactions to two different wavelengths, short and long. Bands were seen at 500 nanometers, corresponding to blue-green, and at 660 nanometers, corresponding to red. Only five of the tested diamonds did not phosphoresce.

"Even though many blue diamonds appear pink or bluish when exposed to ultraviolet light, we found that all blue diamonds do have red phosphorescence.  Unlike the Hope Diamond, some blue diamonds' red light is overpowered by the blue green," says Peter J. Heaney, professor of geosciences at Penn State.

Carbon impurities in a diamond make its color. Boron is the impurity in the blue diamond that makes it appear blue in natural light; blue diamonds have very low levels of nitrogen (the element that makes some diamonds appear yellow). The interaction of the two elements most likely causes the red phosphorescence.

According to a report from Penn State, "When the researchers compared the peak intensities of the 500 and 660 nanometer bands against half the time it took for 660-nanometer light to dissipate, they realized that each diamond had an individual signature. They used the 660 band because the 500 band always disappears faster than the 660. That simple mathematical ratio produces a unique value for every natural blue diamond."

Researches tested three artificial diamonds that were doped with boron. These diamonds looked real to the naked eye, but at 660 nanometers, the synthetic diamonds lacked a peak.

Research on diamonds is specifically important because diamonds make wonderful semiconductors, superior to silicon because they are one of the best conductors of heat.

This specific research led researchers to believe that they could use similar methods to non-invasively fingerprint diamonds. For a long time Heaney's work has focused on these methods because of the problems with conflict diamonds, diamonds sold to support military action against legitimate governments. The aforementioned Penn State report also states, "Understanding the phosphorescence of blue diamonds may make it possible to physically identify individual blue diamonds to add to the current Kimberly Protocols, a paper trail method that ensures diamond come from legitimate sources."