Keshi pearls are formed when the oyster rejects and spits out 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.
What is meant by "implanted nucleus?" Is it talking only about the implanted bead, the implanted tissue, or both?

How can the implanted nucleus fracture and form separate pearl sacs without nuclei?

I read a keshi pearl can form when the bead is rejected, but the implanted tissue remains and it is then coated forming a pearl.