Hi,
This is a piece of nautilus shell. It is sometimes called an "osmena pearl" but it's not actually a pearl.
A blister pearl forms when a hemispherical form is attached to the inside of a shell and nacre is secreted over it. The resulting half-pearl is cut away with some of the surrounding shell and the hemispherical form is left inside it.
In a mabé pearl, however, the half-pearl is cut away from the surrounding shell, the inserted form over which nacre was deposited is removed and the pearl is filled in. The back side is covered, usually with a mother of pearl disk. A mabé pearl is thus an assembled pearl.