Cultured freshwater pearls are one of the birthstones for the month of June. They are produced by mussels and are commonly found in lakes and rivers in China. Most mollusks that produce quality pearls are capable of producing up to 3 pearls in its lifetime. Pearls are formed when a small foreign object or irritant finds its way into an oyster, mussel, or clam. These shelled creatures recognize that there is an irritant and responds by producing a substance called nacre, a protective coating which gives the pearl its iridescent white luster. The longer the pearl is allowed to cultivate, the thicker the nacre and thus increasing its iridescence and value.