Concretion formed by a mollusk consisting of the same material (called nacre, or mother-of-pearl) as the mollusk's shell. It is a highly valued gemstone. Pearls are characterized by their translucence and lustre and by a delicate play of surface colour called orient. The more perfect its shape (spherical or droplike) and the deeper its lustre, the greater its value. Only those pearls produced by mollusks whose shells are lined with mother-of-pearl (e.g., certain species of both saltwater oysters and freshwater clams) are really fine pearls; pearls from other mollusks are reddish or whitish, porcellaneous, or lacking in pearly lustre. Jewelers commonly refer to saltwater pearls as Oriental pearls and to those produced by freshwater mollusks as freshwater pearls.