EXAMINING THE DEFECTS IN NATURALLY IRRADIATED GREEN DIAMONDS: A STATISTICAL APPROACH TO SPECTROSCOPY

Natural green diamonds are highly valued in the gem trade due to their rarity and beauty. The most common cause of green color is the GR1 (General Radiation 1) defect, which can form when diamond is exposed to radiation in the Earth’s crust. Energy from the natural radiation displaces carbon atoms in the diamond lattice, creating vacancies that absorb light primarily in the red portions of the visible spectrum and usually result in a green or blue-green hue. Sometimes, green radiation “stains” occur on diamond surfaces due to exposure to alpha particles from radioactive minerals or fluids. Geologically, these indicators of intense localized damage are important because they record direct interaction between a diamond and its environment. Subtle changes in these stains also record changing geological conditions during diamond residence in the crust.

Many non-destructive spectroscopy methods are used to study diamond defects, but two of the most powerful for gemology are infrared (IR) and photoluminescence (PL). To investigate naturally irradiated diamonds, we have compiled data for 4459 Type Ia (FTIR detectable nitrogen) and IIa (no FTIR detectable nitrogen) gem-quality natural green diamonds. Diamond types were determined from room-temperature IR absorption spectra and PL spectra were acquired using 457 and 514 nm laser excitations at 77 K.

Applying statistical methods to large quantities of diamonds allows for further observations. Non-metric multidimensional scaling and Mann-Whitney U tests indicate that some Type Ia diamonds with green radiation stains have stronger TR12, 484.4, 550.8, and 722.5 nm peak intensities than diamonds without radiation stains. Thus, these defects are likely directly associated with exposure to alpha particles. Type IIa diamonds show different trends due to lower nitrogen contents.

Statistical analysis of spectroscopic data shows that the defects present in natural green diamonds are different when green radiation stains are present, suggesting that the geologic environment in the shallow crust where this radiation exposure occurs has a measurable impact on the structure of green gem diamonds. Characterizing natural green diamonds using their IR and PL spectra may help improve our understanding of the irradiation in Earth’s crust that contributes to their valuable green color.