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A pearl is a hard, glistening object produced within the soft tissue (specifically the mantle) of a living shelled mollusk or another animal, such as fossil conulariids. Just like the shell of a mollusk, a pearl is composed of calcium carbonate (mainly aragonite or a mixture of aragonite and calcite) in minute crystalline form, which has deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque pearls, can occur. The finest quality of natural pearls have been highly valued as gemstones and objects of beauty for many centuries. Because of this, pearl has become a metaphor for something rare, fine, admirable and valuable.
The most valuable pearls occur spontaneously in the wild, but are extremely rare. These wild pearls are referred to as natural pearls. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those currently sold.
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This is a list of auxiliaries of the United States Navy. It covers the various types of ships that support the frontline combat vessels of the United States Navy.
Auxiliary ships which function as hospital ships and as oilers are to be found in their own articles: List of United States Navy hospital ships and List of United States Navy oilers. Escort carriers, amphibious warfare vessels, and some mine warfare vessels were also originally classed as auxiliaries but were later given their own hull classification symbols outside the auxiliary series (which all begin with an 'A'). Links to these and other list articles of similar ships can be found throughout this article.
Yard and district craft also function as auxiliaries but generally are smaller and less capable than their ocean-going counterparts, and so they generally remain in harbors and coastal areas.