Keshi pearls
Keshi pearls are small non-nucleated pearls typically formed as by-products of pearl cultivation. A Japanese word also meaning “poppy” (ケシ, 芥子), it is used in Japanese for all pearls that grew without a nucleus. Originally, keshi pearls referred to those pearls formed when a bead nucleus was rejected. More recently, keshi has been used to refer to second harvest pearls and even to freshwater non-nucleated pearls. However the later usage referring to freshwater pearls is considered erroneous by many leading gem trade associations. Because they have no nucleus, keshi pearls are composed entirely of nacre.
Keshi pearl origins
All of the following can cause a keshi pearl to grow:
An attack on a pearl-producing mollusc by a predator that bores through its shell... similar to a way in which wild natural pearls are formed, except that captive molluscs are more susceptible.
Accidents causing injury to molluscs, for example driving shell fragments into mantle tissue.
Cultivation mishaps, generally separation of the mantle tissue graft from an inserted nucleus... this causes the intended nucleated pearl to fail, and a smaller keshi pearl to grow instead.