How natural pearls are formed

This is how natural pearls are formed :

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.