TEMPERATURE EFFECTS ON RADIATION STAINS IN NATURAL DIAMONDS

Among gem diamonds, one of the most challenging identifications is distinguishing naturally irradiated diamond from laboratory irradiated diamond. In both cases, the GR1 optical center is created, which in high concentrations imparts a greenish color to the diamond. Natural green diamonds are some of the most difficult to certify the natural origin of color.

One of the most important physical features of naturally irradiated diamonds is the presence of radiation stains on the surface of the rough diamond. These radiation stains are generally accepted as being caused by alpha radiation from radioactive minerals or fluids that are in near or direct contact with the diamond surface for an extended period of time. Radiation stains on diamonds transition from a green to brownish color at the temperature range of 550-600^oC. This relatively low temperature range indicates that the presence of green radiation stains could have only occurred after the diamond was near the earth’s surface and no longer exposed to high temperatures.

In this study, thirteen naturally irradiated diamonds with pronounced green radiation stains were chosen for stepwise annealing from 200^oC to 1400^oC. The changes in appearance, UV-Vis absorption spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy (using 325, 457, 488, 514, 633, and 830 nm laser excitation) were assessed after each annealing step. The IR spectra showed these diamonds were principally type Ia but did show a range of A and B nitrogen aggregates and total nitrogen.

Additionally 3 diamonds were subjected to isothermal annealing at 550^oC. The radiation stains were determined to be composed of amorphous carbon networks and their physical and spectral features varied with the extent of radiation damage and nitrogen aggregation.

The major factor for causing the color change in the radiation stains from green to brown is the depletion of GR1. The H3 defect at sufficient concentrations to contribute to the UV-Vis-NIR absorption spectrum is not necessary to create the brown coloration and there was no discernible difference in the color between diamonds which showed the H3 center and those that did not. There was also little discernible difference in the onset of the color change between type Ia diamonds with high or low amounts of nitrogen.