A type of pearl (often hollow and irregular) that is cut away from being attached to the nacreous of the shell of a pearl oyster, and hence having a non-nacreous, flat underside. It is formed whena foreign body (e.g. a grain of sand) has entered the shell and become cemented as an excrescence to the inside of the shell between it and the mantle, causing an irritation that is covered bysecreted nacre except where it is attached; or when a parasite bored its way into the shell and the oyster forms a protective deposit around the entrance.Such pearls are irregularly shaped or somewhat hemispherical. When used in jewelry, the non-nacreous underside is placed in a setting so as to be concealed. Also called a 'chicot pearl' or'wart pearl'.