Keshi pearls are the product of a failed nucleation in either saltwater or freshwater molluscs.The word keshi means "poppy seed" in Japanese and these pearls are often also referred to as "poppy seed pearls."
They develop, when the oyster rejects the implanted nucleus before the culturing process is complete or the implanted mantle tissue fractures and forms separate pearl sacs without nuclei.These pearl sacs eventually produce pearls without a nucleus.
Because there was no nucleus to influence the shaping of the pearl Keshi pearls are small in size and available in various shapes. Consisting of solid nacre, they exhibit a high lustre and often rare orient. Even more, a wide array of colours can be found.
Once especially Tahitian and South Sea Keshis were quite a bargain, showing a shimmering surface quality but never as expensive as "properly" cultured pearls.
Since pearl farmers apply modern technology, Keshi pearls have become a rare gem. Oysters are x-rayed to determine whether or not the oyster has rejected the nucleus. When a nucleus-free oyster is found, it is re-nucleated immediately.