Fresh water pearls come from Muscles(they look like clams but have attachments that hold them to rocks). They are real good and have been cultured by the Chinese. They are of good quality and are competitive with the Salt water pearls that are cultured (seeded artificially) in oysters. Natural pearls are seeded naturally and are not necessarily any better than cultured pearls.
Inside the muscle or oyster shell it is line with the mother of pearl. This is made of the same thing as pearl and looks like it. They can cut this up and make all kinds of jewelry out of it.