Basically an oyster grows just like everything else out there. The mantel is an organ that produces the shell using minerals found in the oysters food supply. The material created is called nacre which lines the inside of the shell. The formation of a pearl starts when a foreign object gets in between the mantel and shell. The oysters natural defense is to cover it up so is to not make it any worse. It starts to cover the irritant with layers over layers of nacre. So basically a pearl is a foreign object covered in layers of nacre. Hope this answers your question!