Large Golden Green Biwa Pearls - with a Hint of GoldPrice is per 15 inch string of these beautiful freshwater pearls. Average largest pearl is about 10mm x 7mm. The least expensive cultured pearls today rival the most expensive natural pearls ever found. 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