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If you have ever clambered across a rocky beach and peered closely into a tidepool or snorkled in deeper waters you will appreciate that Earth's oceans abound in plant and animal life. Amongst the many different types of animals encountered in coastal waters are shellfish which include Bivalve Mollusca (Pelecypoda) such as mussels and oysters. These soft-bodied, invertebrate creatures, if left undisturbed, live out their natural lives cemented to rocks and other hard surfaces. However, both mussels and oysters are valued and enjoyed as human food and are harvested from their natural habitats. An example is Ostrea edulis, a common edible oyster always a delicacy whether it is eaten baked, barbecued, stewed or raw.
Oysters are also valued by humans for their ability to produce pearls. Pearls produced by edible oysters have low luster and resemble pebbles but oysters belonging to a different taxonomic family, the Pinctada, produce fine, highly valued pearls. A pearl is formed in nature when a small object finds its way inside an oyster shell. The resulting irritation induces the mollusk to deposit layers of crystalline forms of calcium carbonate (CaCO3) around the object. The layers are generally made up of aragonite or calcite held together by an organic compound called conchiolin. This combination of calcium carbonate and conchiolin is called nacre and, over a period of several years, the result of adding layer upon layer of nacre is a natural pearl.
Nowadays, almost all pearls used for jewelry are 'cultured.' They are made by inserting into an oyster a nucleus, often a polished bead made from mussel shell, to serve as an irritant. Such cultured pearls are usually harvested three years after the planting and while having less value than a natural pearl look superficially very similar.
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