pearls

Pearls are hard, spherical or irregular organic gemstones produced within the soft tissue of certain mollusks, most notably oysters and mussels. They form as a defense against an irritant, such as a grain of sand or a parasite that enters the shell. In response, the mollusk secretes nacre, a combination of aragonite or calcite crystals and organic layers called conchiolin, which builds up around the irritant until a pearl is formed. Natural pearls are rare; most pearls on the market are cultured, produced by human intervention through pearl farming.

Pearls are classified as saltwater or freshwater, and as natural or cultured. Saltwater pearls include Akoya, Tahitian, and South Sea varieties, produced by oysters in oceans and seas. Freshwater pearls are primarily produced by freshwater mussels in rivers and lakes, especially in China. Cultured pearls are made by surgically inserting a nucleus (usually a bead) and/or mantle tissue to stimulate nacre deposition; freshwater cultured pearls may use tissue-only nucleation. The craft has evolved with Mabe or half pearls, which are cultured against a shell.

The value of a pearl is influenced by size, shape, color, overtone, luster, surface quality, and nacre thickness. Luster refers to the reflective quality of the nacre. Inexpensive or imitation pearls exist, made from glass, plastic, or coated shells; they lack authentic nacre. Pearls require gentle care; they are relatively soft and can be damaged by acids or oils, so they are best stored away from sharp objects and cleaned with a soft cloth.