Irradiation gives pearls a gray color, although the exact shade depends on the type of pearl involved.

In freshwater pearls, the entire pearl turns black to gray. In cultured saltwater pearls, only the nucleus is affected by radiation, turning a black that shows through the nacre and causes the entire pearl to appear dark gray or black.

Radiation treatment can be detected by ultraviolet (UV) spectroscopy, or by microscopic examination of the drill hole, which will reveal the disparity between the surface nacre and the nucleus.