Keshi also refers to saltwater pearls that are without a bead nucleus
because the oyster expelled the bead.
When the bead was implanted, a bit of mantle tissue was also implanted-- it is the mantle tissue that forms the pearl sac. If the bead is expelled the sac remains. The pearl farmer may X-ray the oyster, discover there is no bead, and re-implant the oyster with a new bead, but if not, a keshi results.
Here are my Tahitian keshi (I think you saw them when we got together a few years ago):
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