NATURAL RADIATION STAINS IN DIAMONDS: RATES OF COLOR CHANGE

Gemologists often use radiation stains on diamonds as tools for color origin identification. These green or brown spots provide evidence that diamonds were exposed to radioactive fluids or mineral grains. Experiments have shown that green stains permanently turn to a brown color at ~550–600^oC, making them useful indicators of heat exposure and restricting the depth of radiation exposure to shallow depths in the Earth’s crust. While the green color is known to be caused by GR1 absorption (lattice vacancies) and that high temperatures cause the GR1 absorption to decrease thus allowing the underlying brown-colored absorption to be observed. However, the cause of the brown color remains unclear although it is presumed to be related to vacancy clusters.

In addition to the color mechanism, little is known about the rate of color change for the stains. Although they change within a few seconds to minutes at 600^oC, can this change occur at much lower temperatures when exposed for extended time periods? Knowledge of the rates of change at different temperatures is critical for proper use of the stain color as an indicator of heat exposure, particularly for understanding natural geological processes where diamonds can reside in low temperature alluvial environments for millions of years. Many diamonds with both green and brown stains are thought to have never resided deeper than a few hundred meters since eruption, making it difficult to reconcile the need for such elevated temperatures to change the stain color.

In an effort to better understand the color change of radiation stains, we performed two types of experiments. First, several green-stained diamonds were heated from room temperature to various peak temperatures and held for several hours while being continuously imaged to try to quantify the rate and actual color-space variation occurring during the change. The second phase of our experiment was to collect UV-Visible-NIR absorption spectra while heating green-stained diamonds to the same range of temperatures. These data provide insight into the defects responsible for brown color in the modified radiation stains.

Together, the two experiments can provide needed information about the reliability of using the color of radiation stains as a tool in identification of natural or treated color origins for gem diamonds.