Pair of baroque, natural oyster pearls measuring 13.8 carats
Natural pearls are formed randomly and really are simple accidents of nature. When a certain type of irritant, such as a parasite, becomes lodged in the tissue of a mollusk, the animal responds by secreting a calcium carbonate substance called nacre to coat the intruder and protect the mollusk. Over a period of several years, this build-up of nacre forms a natural pearl.
Natural pearls of any commercial value or desirability are extremely rare. Instead, since the early part of the 20th century, cultured pearls have supplanted natural pearls as the most common and available pearls.