A natural gemstone formed when a oyster is irritated by a substance that gets into its shell.

 

If the irritation is a naturally occurring grain of sand, it is an Oriental pearl. If it is produced by purposefully inserting a mother-of-pearl bead, a cultured pearl is formed.

 

A pearl that forms attached to the shell is a blister pearl, while a pearl that forms a half dome is a mabe (pronounced mah-bay) pearl. Pearls that are irregularly shaped rather than round are referred to as baroque.