A 'pearl' that grows on the interior surface of a mollusk's shell.
A blister pearl results when a shape is glued to the inner shell and the nacre is allowed to form over it. When harvested, any interesting shape may be carved around the dome to incorporate into jewelry designs.
A pearl that forms attached to the shell.
A blister pearl (also called a bouton pearl) is a pearl that developed attached to the inside of a mollusk's shell. This type of pearl must be cut off the shell, and is therefore hemispherical. Because of their shape, blister pearls are mostly used for earrings.