"Then again, a pearl is a kind of pathology—or at least, a reaction to one. It’s the mollusk’s defense against an invader, whether it’s a speck of grit or a parasitic fluke. The mollusk enrobes its enemy slowly in layer after layer of gleaming nacre; the pearl becomes the invader’s tomb. From the centers of pearls, scientists have exhumed tiny crabs, tapeworms, snail shells—even, sometimes, entire fish. In this way, pearls have less in common with other precious gems and more in common with pimples—or with the gnarled galls that swell from the trunks of wasp-infested oaks."
So every pearl has a little creature entombed in the center? Wow, never expected that was how pearls were formed.