Pearls have been highly valued as gemstones since antiquity, and their allure and beauty has always been magnified by the difficulty of obtaining them in early times. Unlike most other gemstones which are minerals, Pearls are organic and are formed by living organisms. They are generally formed within the soft tissue layer of mollusks such as oysters and mussels. Pearls are composed of calcium carbonate (in the form of Aragonite) and organic conchiolin that build up as concentric layers as they are secreted by their host.