(MOP0007) Mother of Pearl, triangular shape.
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.