Keal Byrne

The Smithsonian’s National Gem Collection includes the Hope Diamond and an assortment of other significant fancy-colored diamonds, providing a unique opportunity to conduct detailed and sustained studies on an unprecedented selection of these rare and valuable stones. We present an overview and recent results from our work on pink, blue and chameleon diamonds.

Boron causes the blue color of the Hope Diamond and other type IIb diamonds, but scarcity, high value, and the low concentration of B has inhibited B analyses of natural IIb diamonds. We used FTIR and ToF-SIMS to measure concentrations and distributions of B in the Hope and other blue diamonds. ToF-SIMS analyses gave spot B concentrations as high as 8.4 ± 1.1 ppm for the Hope Diamond to less than 0.08 ppm in other blue diamonds and revealed strong zoning of B in some diamonds, which was confirmed by mapping using synchrotron FTIR. Boron is also responsible for the phosphorescence emissions of IIb diamonds, at 660 nm and 500 nm; the emissions are likely caused by donor-acceptor pair recombination processes involving B and other defects.

Approximately 50 type I natural pink diamonds were compared using UV-Vis, FTIR, and CL spectroscopies. All stones exhibit pink color zoning, ~1µm thick [111] lamellae, in otherwise colorless diamond. The pink diamonds fall into two groups: 1) those from Argyle in Australia and Santa Elena in Venezuela, and 2) those from other localities. TEM imaging from FIB sections revealed that twinning is the likely mechanism by which plastic deformation is accommodated for the pink diamonds. The deformation creates new centers, including the one responsible for the pink color, which remains unidentified. The differences in the plastic deformation features for the two groups might correlate to the particular geologic conditions under which the diamonds formed.

Fluorescence and thermoluminescence experiments on natural chameleon diamonds reveal that an emission band, peaking near 556nm, may be stimulated via a number of different mechanisms. We discuss the implications of our observations for the electronic structure of the 556nm-fluorescing defect center, and the connections to the unidentified color center responsible for chameleon color changes.

Some diamonds reversibly change color in response to the influence of temperature and light. An understanding of how these processes occur in each unique case has both scientific and commercial impact for the identification and authentication of diamonds. The underlying physical systems that drive these diamond color changes may also manifest changes in other measurable properties, such as luminescence. We have investigated luminescence in two classes of diamond noted for displaying color changes: (1) pink/brown diamonds, and (2) 'chameleon' diamonds. The defect centers responsible for diamond color in these cases are not well understood; we examined luminescence behaviors seeking to learn more about electronic transitions underlying the diamond color, and color-change. Some pink/brown diamonds demonstrate two distinct phases of photochromism--one induced by excitation in the visible domain, and one sensitive to ultraviolet and infrared light. The latter induces luminescence from diamond defects that do not otherwise interact with the pink color center. A new survey shows that this luminescence is visible in a wide range of pink/brown diamonds, and involves a larger number of defects than had been previously observed. We will discuss the implications of these results for the current understanding of pink diamond color, and its relationship with brown color in diamonds. 'Chameleon' diamonds show characteristic fluorescence peaking near 560 nm. This band is strongly temperature-sensitive, suggesting a possible association with the diamonds' thermochromism. 560 nm-band phosphorescence following UV exposure suggests a thermal release of trapped charge carriers from several distinct traps. Infrared light can also empty the trap states, and may be used to measure lifetimes of long-lived states.

Organizer: 中国珠宝玉石首饰行业协会;
Argyle pink diamonds are type Ia and exhibit color-changing effects when illuminated with blue or UV light with a threshold at 465nm.The level of color loss is dependent on the wavelength of illumination, increasing with shorter wavelengths while the intensity of light determines the rate of color change. Color restoration is achieved with light between 570nm and 465nm.Below 300nm the color is partially restored unless the diamond is illuminated with IR light. An energy level diagram is proposed which includes single N and brown color centres.