Pearls are hard, glistening objects that are produced within the soft tissue of a living shelled mollusk or another animal, such as fossil conulariids. They consist of calcium carbonate in minute crystalline forms, just as the shells of mollusks, which have deposited in concentric layers. The calcium carbonate is mainly aragonite, or a mixture of aragonite and calcite. The ideal pearl is perfectly round and smooth, but many other shapes can occur, and pearls with these other shapes are called...