Unlike gemstones that are mined from the earth, a living organism produces a pearl and, in fact, their very existence is a freak of nature. A pearl is formed when an irritant, such as a parasite or piece of shell, becomes accidentally lodged in an oyster's soft inner body, causing it to secrete a crystalline substance called nacre, which builds up around the irritant in layers until a pearl is formed. Cultured pearls are formed through the same process, the only difference being that the irritant is implanted in the oyster rather than entering it by chance.
