Keshi pearls are created accidentally by the mollusk. It is either a second growth byproduct after a nucleated pearl has matured or grown after a nucleus is rejected by the mollusk. They are total nacre.
Keshi Pearls are a little known form of, what I like to describe as, "pearls gone wild". Keshi Pearls are smaller and grown as a second growth of a cultured pearl or if the first nucleation is rejected.