The irradiation method of treating pearls has different effects on Freshwater and Saltwater Pearls. For Freshwater pearls, the nacre, rather than the nucleus (since freshwater pearls are mostly pure Nacre), is affected by the gamma rays. They in turn can become very dark black or brown from this treatment. The freshwater pearls are also known to develop a metallic sheen and/or a highly sought after iridescent orient surface from this treatment. Beautiful black freshwater pearl jewelry can be produced from these pearls.
For Saltwater Pearls, the gamma rays have little affect on the Nacre however it has a huge affect on the nucleus of the pearl since the nucleus is source from Freshwater Mussels. The gamma rays darken the nucleus of the pearl, making saltwater pearls appear blue or gray. This color filters through the nacre to the surface to give the pearl a shimmering affect.