It's a shell with blister pearls. Pearls are cultured this way-- a form is attached to the inside of the shell, and the oyster is returned to the water. The oyster then lays down nacre over the form.
At harvest, the shell can be cut away a small distance from the pearl and sold as a blister pearl with shell around it. like this:
Alternatively, the shell can be cut away right around the pearl, and the shell and form removed, leaving just the nacre, which is hollow, like a cup. The nacre is then filled, and a mother-of-pearl backing is placed over the back to cover it. These are sold as mabe pearls.