Submitted by Buck Nacre
Way back when, someone whose identity is lost to pearling history declared that a natural pearl forms when foreign matter invades a shelled mollusk’s soft tissue, and the mollusk progressively coats the invader with shell material to sooth the irritation it causes. That explanation has been repeated so often, it’s taken as true. But both common sense and close analysis demonstrate it’s false.
First, common sense: If soothing irritation were the mollusk’s purpose in coating the foreign matter with shell material, why would it continue coating once enough layers were applied to give the irritant a smooth surface?
Second, close analysis: Foreign matter gets inside the mollusk and either lodges in the mantle tissue where the epithelial cells that produce shell material are, or it picks up epithelial cells on its way to settling somewhere else in the mollusk’s soft body. Whether in the mantle or elsewhere, the epithelial cells continue doing what they’re genetically programmed to do -- produce shell material. But rather than continuing to build a shell, the cells form a pearl sac and create a pearl that encapsulates the invader. A cultured pearl with a bead as its core is produced by adapting this process.
For more detail, see “Pearl Production” by Joseph Taylor and Elisabeth Strack in The Pearl Oyster
, edited by Paul Southgate and John Lucas, and published by Elsevier.