Pearls, the birthstone for June, have long been considered among the most valuable items one could possess. A string of matched natural pearls can be considered priceless. Now nearly all pearls are farmed, or cultured, making them more widely available.

A pearl is formed when some sort of small object, typically a parasite or piece of organic matter, becomes embedded in the tissue of an oyster or mollusk. In response, the mantle tissue of the mollusk secretes nacre. Chemically speaking, this is calcium carbonate and a fibrous protein called conchiolin. As the nacre builds up in layers, it surrounds the irritant and eventually forms a pearl. It is a myth that a grain of sand can cause a pearl to form as nacre will not adhere to inorganic substances.

Natural pearls are those pearls which are formed in nature, more or less by chance. Cultured pearls, by contrast, are those in which humans take a helping hand. By actually inserting a foreign object into the tissue of an oyster or mollusk, pearl farmers can induce the creation of a pearl. The same natural process of pearl creation takes place.

Fine gem-quality saltwater and freshwater pearls can and do sometimes occur completely naturally, but this is rare. Many hundreds of pearl oysters or pearl mussels have to be gathered and opened, and thus killed, in order to find even one pearl, and for many centuries that was the only way pearls were obtained. This was the main reason why pearls fetched such extraordinary prices in the past. In modern times however, almost all the pearls for sale were formed with a good deal of expert intervention from human pearl farmers.

Imitation or fake pearls are also widely sold in inexpensive jewelry, but the quality of the iridescence is usually very poor, and generally speaking, fake pearls are usually quite easy to distinguish from the real thing.

Pearls have been harvested, or more recently cultivated, primarily for use in jewelry, but in the past they were also stitched onto lavish clothing, as worn, for example, by royalty. Pearls have also been crushed and used in cosmetics, medicines, or in paint formulations.