Black Akoya pearls are dyed to achieve this color. The dyeing process leaves pearls that are incredibly uniform in color, and very saturated.
Black Freshwater (China) and black Akoya pearls (Japan) are produced with dyes and treatments. Black color does not exist naturally in these two types of pearls.
Jewelry technicians employ dyes or irradiation to transform light colored pearls into black pearls. One treatment involves an Akoya or Freshwater pearl being dipped or soaked in a silver nitrate solution which darkens the pearl's nacre like a dye. Sometimes irradiation is also used. This process employs gamma rays that darken the nucleus of the pearl. While these processes are artificial, they are considered permanent and often produce fantastic body colors that many jewelry lovers find appealing.
Color treated Freshwater pearls are near jet black pearls with blue, green, violet, copper and sometimes even peacock overtones. Similarly, color treated Akoya pearls are deep black pearls with strong green or blue overtones.