Since most people are somewhat familiar with the basics of pearl formation, we understand the analogy of taking an unpleasant event or situation and using it “to form a pearl.” Natural freshwater pearls form in mussels for the same reasons that saltwater pearls form in oysters. A foreign body becomes lodged in the mollusk, and in order to reduce the irritation on its soft body, the mollusk coats the object with the same secretion it uses for shell-building. The shell-building secretion is called nacre and is the same lustrous, shimmering material as mother-of-pearl. The foreign body can be something sharp, ex. sand, or a parasite of some type. This explains the reason for the irregular shape of natural pears as compared to the uniform shapes of cultured pearls.
The types of mollusks that produce pearls are mussels, oysters, clams, conch, and abalone. A natural pearl forms without any “help” from man, whereas a cultured pearl usually forms because some foreign object along with a small piece of mantle tissue has been implanted into the mollusk. This object is usually a bead made of shell, because this is close to the density of a natural pearl. (Think of a strand of cultured pearls as compared to an inexpensive strand of faux pearls of the same dimensions: the cultured pearls will feel heavier and more substantial.) In the case of freshwater mussels, a pearl will form with just an implanted piece of mantle. So cultured freshwater pearls are made of solid nacre just as natural saltwaters and natural freshwaters are. The nacre from the freshwater mussels and saltwater oysters is very similar chemically and structurally.