Chameleon diamonds: Thermal processes governing luminescence and a model for the color change

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.