Are there black colored pearls? Are they naturally black in color or treated in order to attain that color?

“Pearls are often white or cream, but the color can vary quite a lot according to the natural color of the nacre in the various species of mollusk used. Thus pearls can also be black, or various pastel shades. In addition, pearls (especially freshwater pearls) can be dyed yellow, green, blue, brown, pink, purple, or black.”
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We bought our daughter a strand of black pearls and a pair of black pearl earrings in Hawaii last summer. They are beautiful!