Stimulus, reaction, formation
The formation of a natural pearl begins when a foreign substance slips into the oyster between the mantle and the shell, which irritates the mantle. It’s kind of like the oyster getting a splinter. The oyster’s natural reaction is to cover up that irritant to protect itself. The mantle covers the irritant with layers of the same nacre substance that is used to create the shell. This eventually forms a pearl.
“The formation of a natural pearl:.”
Image courtesy of https://imgur.com/gallery/yMVZf.