Pearl Mother of. The shell, not of the pearl oyster, but of another kind of oyster, the inside of the shell of which is very smooth and polished, and of the whiteness and water of pearl itself. The shell has the same lustre on the outside, after the outer coat, or lamina, has been removed by aqua-fortis and the lapidary's mill. It is used for the handles of knives, inlaid work, etc.