How was a pearl formed?

"An accident could happen to these oysters, a grain of sand could lie in the folds of muscle and irritate the flesh until in self-protection the flesh coated the grain with a layer of smooth cement. But once started, the flesh continued to coat the foreign body until it fell free in some tidal flurry or until the oyster was destroyed. For centuries men had dived down and torn the oysters from the beds and ripped them open, looking for the coated grains of sand."