How a Pearl Forms

The usual account has a grain of sand working its way into an oyster, which then coats the irritant in layer after layer of the same material it uses for its shell. [A] The account is almost right, and wrong in the part that is easiest to remember. [B] Sand is rarely the cause. [C] A pearl in the wild is far more often built around a fragment of the animal's own tissue, or around a parasite that has burrowed through the shell. [D]

The difference matters because it explains why pearls are rare. Sand passes through a shellfish constantly and is expelled without ceremony. What produces a pearl is not an object but a piece of the tissue that makes shell, carried into the body where it does not belong. That tissue keeps doing its work, laying down layers in a place where they enclose nothing useful, and the animal has no way of removing it.

Cultured pearls follow the corrected version rather than the old story. A worker inserts a bead, but the bead alone produces nothing; a small piece of tissue from another animal has to be placed beside it. The bead is only a shape to build on. Whether a pearl appears at all depends on the tissue, which is why the operation takes training and why so many of the animals treated produce nothing worth selling.