Large Shiny Enchanting Silver Pearl Nugget BeadsPrice is per 15 inch string of these lustrous pearls. Largest in the strand is about 10mm x 9mm x 9mm about 39 pearls to the string. Cultured pearls Most pearls these days are cultured. These natural freshwater pearls occur in mussels for the same reason saltwater pearls occur in oysters. Foreign material inside a mussel can't be expelled. To reduce irritation, the mollusk coats the intruder with the same secretion it uses for shell building, nacre.