GEM DIAMONDS THAT TEMPORARILY CHANGE COLOR (CHAMELEON DIAMONDS)

Chameleon diamonds are gem diamonds that are famous for their remarkable ability to temporarily change color after heating or prolonged storage in darkness. The stable green body color changes to intense yellow or orangy yellow and then reverts back to the original hue as the stone cools or is exposed to light. Despite considerable documentation, the origin of chameleon diamonds and the mechanisms responsible for the color change remain unclear. In order to better understand their origin, we examined internal structures in more than 75 of these rare stones using magnification, optical spectroscopy, and high energy UV fluorescence imaging. Quantitative spectroscopic analysis was performed on a double-sided polished plate of chameleon diamond to investigate the thermochromic behavior. Fluorescence imaging revealed that the internal structure of these diamonds is complexly zoned. A few areas exhibited crystal growth patterns, but most zones displayed irregular morphologies with edges that resembled resorption features. Spectroscopic data showed heterogeneous distributions of nitrogen complexes and hydrogen within the zoned structure. Photoluminescence analysis also indicated the presence of nickel, a commonly incompatible element, in all samples. Some of the diamonds contained mineral inclusions consisting of garnet, omphacite, and/or apatite. All of these data and observations strongly suggest that chameleon diamonds formed in high pressure metamorphic environments in the Earth. To explore the thermochromic behavior, absorption spectra in the visible light range were collected at different temperatures on a 1.48 mm thick plate of chameleon diamond. At room T, two broad absorption bands at ~480 nm and ~830 nm created a transmission “window” in the range 530-540 nm, which was responsible for the green body color. With increasing T, the ~830 nm band remained unchanged, but the ~480 nm band broadened. As a result, the “window” shifted to longer wavelengths. At 150-200°C, the “window” was at 560-570 nm and the stone appeared yellow. By 450°C, the “window” achieved a maximum shift to ~590 nm, generating an orangy yellow hue. This experiment revealed for the first time that broadening of the ~480 nm absorption band with increasing temperature is the main cause of the color change in chameleon diamonds.