Temperature effects on radiation stains in natural diamonds

The green coloration of natural diamonds typically results from exposure to natural irradiation. This creates the GR1 optical center and in many diamonds, surficial damage, principally due to alpha radiation, which helps verify natural origin. In this study, 13 naturally irradiated diamonds with pronounced radiation stains were stepwise annealed from 200 degrees C to 1400 degrees C and the changes in color and defects were documented by photomicrography and spectroscopy. Additionally 3 diamonds were subjected to isothermal annealing at 550 degrees C. The radiation stains correlated with radiation-damage Raman peaks - a broad and shifted diamond Raman peak and radiation-related peaks at 1500 and 1640 cm(-1). The color transitioned from green to brown after heating to 550-600 degrees C and the stains were essentially decolorized at 1400 degrees C. Confocal Raman depth profiling showed that the depth penetration of the radiation stain was about 10-15 mu m into the diamond and this depth profile was distinctly different from depth profiles of ion-irradiation stains generated in a laboratory. (C) 2016 Elsevier B.V. All rights reserved.