How are pearls created by oysters?

Pearls are made of nacre, which on its own is commonly known as "mother of pearl" (because it is the substance from which true pearls are birthed). Oysters do not actually make pearls deliberately, as it were; pearls are a by-product of the oysters' need to protect themselves from invaders and irritants. For example, if a piece of grit or sand makes its way into the oyster's sensitive internal area, it will begin to coat that piece of grit in layer upon layer of nacre in order to shield itself from the irritation that would otherwise result. Sometimes, when pearls are made deliberately by "pearl farmers," an irritant will therefore be introduced into an oyster's shell to encourage this process. Essentially, however, the process of an oyster making a pearl is similar to the process by which our eyes begin to water when a piece of grit touches the soft surface of the eyeball and needs to be washed away.
https://www.livescience.com/32289-how-do-oysters-make-pearls.html