When pearl farmers open a mollusk to culture a pearl, they sometimes discover a pearl that has occurred naturally. While some insist that any pearl found in a farming operation is cultured, natural pearls do form in mollusks before they are opened for the first time.
This is not the same as keshi, small baroque pearls that form after mollusks have been grafted. These small treasures can't be counted as natural.
This Pteria sterna, or "Rainbow Lip" oyster is held in a grafting clamp...