Gamma ray irradiation darkens the nucleus and results in darker pearls, sometimes dark enough to resemble natural-color black South Sea pearls. The advantage, of course, is they can be sold for much less. Irradiation also enhances the display of iridescent colors (called orient) in some pearls. In either case, the pearls retain no radioactivity, thus, the enhancement is considered harmless. Most experts believe this treatment is permanent.