Pearls are one of the few varieties of gem materials with an organic basis, along with amber and jet. Pearls are "manufactured" by marine oysters or fresh water mussels who ingest water and food by means of inhaling through rudimentary mouths, which sift through the contents. Occasionally a grain of sand or other irritant gets lodged in the animal and becomes encrusted with the shell-secreting material known as nacre. Layer upon layer of nacre is added over time, forming what we term a pearl. Those of greatest value are those from the South Seas of Australia and French Polynesia, imparting the best color, shape, and size. Originally harvested by men who would dive deep into the ocean, holding their breath for as much as seven to eleven minutes at a dive, this method became outdated due to the dangers as well as the introduction of cultured pearls. Several good books about pearls and web sites are available to the interested parties.