A pearl is a gemstone characterized as rare, precious and valuable. It is the result of an irritant (namely, a grain of sand) entering the mantle of a living mollusk (typically, an oyster.) In essence, the oyster is taking something otherwise irritable and turning it into a thing of beauty and value. While Hester's clandestine relationship, once exposed through her pregnancy, is something that causes irritation and anger throughout the community, the arrival of her precious daughter Pearl brought her the joy that only a parent can understand. Pearl was her pride and joy, the beautiful thing that resulted despite her sin.