How is a pearl formed?

I don't recall the book going into any detail on how a pearl is actually formed. You can check out this link that explains the science behind it:

I'm a big pearl enthusiast but I also make sure that I buy from an ethical business that sells pearl jewelry (DAMA is a good ethical brand https://www.stylewithdama.com/ ). I've recently read an article about how and why oysters make pearls, check them out here