Pearls come from what are called long mollusks. They are made of calcium carbonate. I read that they are a bit like a clam shell. They form in layers from what I read and there are different kinds.
Some pearls are very valuable. They are the ones that get locked in safes when not being worn. They are considered gemstones.
All of the pictures are from Wikimedia Commons.
This is a picture of a pearl diver in Japan by, User:Fg2.