A natural pearl is formed when a microscopic foreign object becomes trapped inside a mollusk shell. The object can irritate the tissue, so it is coated in layer after layer of nacre, the same material that forms the iridescent coating of bivalve shells.

It is these consecutive translucent layers that cause the optical effects of pearls, as some light is reflected from various layers within.

The resulting pearls can form in a wide variety of shapes, sizes, and colors. The varieties depend on the properties of the seed object and the species of mollusk.

The quality and value assigned to a pearl is based on its size, shape, color, quality of surface, orientation and luster. Of course, this rating is a human invention.

In nature, all pearls represent a function that mollusks have evolved to perform to keep themselves safe. That process is the real winner here, of which all pearls are equal byproducts.