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.

640px-Ama2 User Fg2 WCThe next two pictures are of Keshi pearls   by, Mario Cateb. The second picture is a close up.

Keshi_pearls Mario Cateb WCKeshi_pearls_-_detail Mario Cateb WCFreshwater pearls  by, Jenifergadlioni.

Freshwater_pearl_texturedloose Jennifergaglioni WCA cameo  by, Clie20.

445px-Cameo_Augustus_Cdm_Paris_Chab190 Clie20 WCGold and pearl necklace  by, Mario Cateb.

Gold_and_pearls_neckband MArio Cateb WCThis last picture is of a Diademe  de Imperatrice Eugene, from the Louvre Museum  by, dalbera from Paris, France. I couldn’t make the accents on my keyboard.

Diadème_de_limpératrice_Eugénie_(musée_du_Louvre)_(7166066743) dalbera from Paris France WC