Pearls are truly wonders of the sea. Formed by oysters and other mollusks, their luster captivates the human eye.
It is a myth that pearls are formed by a grain of sand. Sand gets into oysters all the time. Natural pearls are actually formed when a piece of shell or a small creature gets into the oyster and irritates it. The oyster responds by encasing the irritant in nacre, the shiny substance that also coats its shell, thus rendering it smooth. Cultured pearls are formed around a bead carved from a shell from a mussel in the United States.
Pearls are graded based on color, blemishes, luster, nacre thickness, roundness and matching. The AAA, AA, A grades that you see are not a uniform grading system and vary greatly by company, so one company's B grade may be a different company's AA grade.
It is important to purchase pearls from a trustworthy store with a knowledgeable staff and to look at luster and blemishes rather than letter grades.