What is a pearl

Do you mean a general deffinition? Pearl: a hard, lustrous spherical mass, typically white or bluish-gray, formed within the shell of a pearl oyster or other bivalve mollusk and highly prized as a gem.

Kino dives for oysters from his canoe, hoping to find a pearl he can sell to pay the doctor. He finds a very large oyster which yields an immense pearl, and which he dubs "The Pearl of the World".