Keal Byrne

To date, the eponymous color-changing behavior of chameleon diamonds lacks an explanation in terms of an identified diamond defect structure or process. Well known, however, is that this color-change is driven by the influence of both light and heat. In this paper, we present observations of how luminescence emission in chameleon diamonds responds to temperature changes and optical pumping. Fluorescence, phosphorescence, and thermoluminescence experiments on a suite of natural chameleon diamonds reveal that a specific emission band, peaking near 550nm, may be stimulated by several different mechanisms. We have observed thermal quenching of the 550nm emission band with an activation energy of 0.135eV. The 550nm band is also observed in phosphorescence and thermoluminescence. Thermoluminescence spectra suggest the presence of low lying acceptor states at 0.7eV above the valence band. When excited with 270nm light, we observe emission of light in two broad spectral bands peaking at 500 and 550nm. We suggest that the 550nm emission band results from donor—acceptor pair recombination (DAPR) from low lying acceptor states at ca. 0.7eV above the valence band and donor states approximately 2.5 to 2.7eV above the valence band. We do not identify the structure of these defects. We propose a speculative model of the physics of the color change from ‘yellow’ to ‘green’ which results from increased broad-band optical absorption in the near-IR to visible due to transitions from the valence band into un-ionized acceptor states available in the ‘green’ state of the chameleon diamond. We report near-IR absorption spectra confirming the increased absorption of light in the near-IR to visible in the ‘green’ when compared to the ‘yellow’ state with a threshold at ca. 0.65eV, supporting the proposed model.
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•Measured fluorescence and phosphorescence of chameleon diamonds•Measured near IR absorption of a chameleon diamond in it ‘green’ and ‘yellow’ states•Proposed a model for the physics of the color change in chameleon diamonds

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.

Irradiation of natural pink and brown diamond by middle-ultraviolet light (photon energy ≥ 4.1 eV ) is seen to induce anomalous fluorescence phenomena at N3 defect centres (structure N3−V). When diamonds primed in this fashion are subsequently exposed to infrared light (even with a delay of many hours), a transient burst of blue N3 fluorescence is observed. The dependence of this IR-triggered fluorescence on pump wavelength and intensity suggest that this fluorescence phenomena is intrinsically related to pink diamond photochromism. An energy transfer process between N3 defects and other defect species can account for both the UV-induced fluorescence intensity changes, and the apparent optical upconversion of IR light. From this standpoint, we consider the implications of this N3 fluorescence behaviour for the current understanding of pink diamond photochromism kinetics.

The Blue Moon diamond, discovered in January 2014 at the historic Cullinan mine in South Africa, is of significance from both trade and scientific perspectives. The 29.62 ct rough yielded a 12.03 ct Fancy Vivid blue, Internally Flawless gem. The authors were provided the opportunity to study this rare diamond at the Smithsonian Institution before it went on exhibit at the Natural History Museum of Los Angeles County. Infrared spectroscopy revealed that the amount of uncompensated boron in the diamond was 0.26 ± 0.04 ppm, consistent with measurements of several large type IIb blue diamonds previously studied. After exposure to short-wave ultraviolet light, the Blue Moon displayed orange-red phosphorescence that remained visible for up to 20 seconds. This observation was surprising, as orange-red phosphorescence is typically associated with diamonds of Indian origin, such as the Hope and the Wittelsbach-Graff. Time-resolved phosphorescence spectra exhibited peaks at 660 and 500 nm, typical for natural type II blue diamonds. As with most natural diamonds, the Blue Moon showed strain-induced birefringence.

Position annihilation measurements of natural brown type I diamonds revealed that they contain monovacancy-type defects and large vacancy clusters. The latter defect type was absent in clear diamonds. Both the intensity of the positron vacancy cluster lifetime and the S-parameter were found to increase with the intensity of brown colouration determined by optical absorption measurements. No correlation was found between the nitrogen content and the positron annihilation parameters.

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.

The colour centre responsible for pink colouration in natural diamonds has not yet been identified. This paper presents optical and Infrared (IR) microspectroscopy in order to identify the degree of spatial correlation between the IR absorption peaks and the pink colouration of single-crystal natural Argyle diamond. The spatial distribution of the nitrogen B-centre (N–V–N3) is found to be anti-correlated with the pink colour. In contrast, no change is observed in any IR feature (including the B-centre) in response to driving the photochromism of natural pink diamond. We use these results to support a proposed mechanism for production of the pink colouration.

Naturally occurring pink diamonds include defect centers with properties that differ greatly from those of commonly synthesized diamond centers. The pink diamond color-center demonstrates optically-controllable photochromism which is stable at ambient temperatures. The nature of this defect and the origin of the photochromism are yet to be explained. In this work we show that the photochromic behavior can be explained by competing photoionization processes at multiple defect centers in response to an applied optical pump. Our approach quantitatively explains the dependence of both the response rate and the resulting color on the pump wavelength. From measurements of the photochromic response we are able to extract parameters that describe the ionization cross-sections of the involved centers.