Pearls are organic gems formed in the soft tissue of giant clams, pearl oysters and other molluscs. When a microscopic object, such as a parasite or shell fragment, gets trapped in the soft tissue of the mollusc, it irritates the tissue’s epithelial cells. In response, the mollusc envelopes the irritant in a sac and a crystalline substance called nacre is secreted, which coats the irritant. Over time, a pearl is formed (Figure 1).
Nacre, also known as mother of pearl, is composed of the mineral aragonite, a form of calcium carbonate (CaCO3, see Figure 2 and CHEMISTRY REVIEW, Vol. 20, No. 3, pp. 8–11). The combination of inorganic aragonite in an organic protein matrix (namely conchiolin) means that nacre is a composite material.
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