PINK RADIATION DAMAGE IN NATURAL GEM DIAMOND

Areas of radiation damage on a gem diamond, often termed by gemologists as “radiation stains”, occur as green or brown patches of color that penetrate into a diamond’s surface to a depth of 20 to 30 micrometers. These radiation stains are damage in the diamond lattice caused by alpha particles emitted by adjacent radioactive minerals (or salts precipitated by fluids) during radioactive decay of uranium or thorium isotopes. It is widely believed that the stains are green when initially created (due to isolated vacancy defects) and can change color to brown (due to underlying vacancy clusters) when heated to anneal the isolated vacancies. This change occurs at ~500 degrees C in the lab, but likely at lower temperatures over long periods in alluvial environments in the earth’s crust.

Until recently, radiation stains were only known to occur as green or brown. However, recent discoveries have revealed areas of radiation damage with pink color. The gem diamond containing the pink stain is type Ia with very high concentrations of A-aggregated nitrogen and hydrogen impurities along with measurable amounts of isolated nitrogen (C-centers). A yellow zone of body color in the stone coincides with the pink stain, strongly suggesting the occurrence of C-centers in that region. The stain itself contains very high concentrations of NV- defects that are responsible for the pink color. Very little GR1 (vacancy) occurs in the diamond, but interstitial defects are present adjacent to the stain. Presumably this pink stain started as a green one and turned pink as vacancies became mobile and then trapped at abundant C-centers during long, low-temperature annealing in the earth. This unusual occurrence reminds us that natural diamond is very heterogeneous and just the right combination of defects can produce unexpected results.