Radio-coloration of Diamond

This study investigated the green and orangy brown spots that commonly occur on the surface of rough diamonds and which are caused by external irradiation by helium ions in the form of alpha particles. These ions are believed to be generated by the decay of radionudides in radioactive minerals that were located close to a diamond in irs geologic past. Helium ions generated in the U- and Th-decay chains have energies in the 4.0-8.8 MeV range; their travel distances in diamond arc up tO 29 1-1m. Depending on rhe sizes (mostly unknown) of rhe radiation sources and their proximity to the diamond's surface, helium irradiation typically results in blurred round spots with green color. These spots are usually deformed semi-spheres with fuzzy boundaries. With moderate-temperature heating, the green radio-coloration turns orange-brown. Raman measurements revealed that these richly colored green and brown spots represent elevated levels of radiation dan1age. The same observation was made on spots produced by the irradiarion of diamond plates with 8.8 MeV He ions in a tandem accelerator, foUowed by annealing experiments. The transformation from green to brown was accompanied by relatively low degrees of structural reconstitution.
Our experiments showed that irradiation with about l x l 0¹⁶ He ions per cm₂ is needed to produce strong radio-coloration. This in rum leads us to conclude that radio-coloration on the surface of natural stones must be a long-tenn process: A typical spot has a size on the order of 1,000 fU112, requiring about 1 x 10¹¹ helium ions. If the helium irradiation emanated, for instance, from a 5 fUll uraninirecrysral {>80wr.% U), it would rake more than 10 million years to accumulate l x 10¹¹ helium ions in the neighboring diamond In the case of a lO fUll zircon crysra1 containing 1,000 ppm of uraniurn, that same alpha dose would require more than 2 billion years, which seems implausible. Therefore, radio-coloration is likely due to the action of small grains of strongly radioactive phases that lay adjacent to the diamond for at least several million years.