Mad About Pearls

A pearl is a hard object produced within the soft tissue of a living shelled mollusk. A small lustrous sphere of calcium carbonate forms around a grain of sand in a living organism such as an oyster. The calcium carbonate is deposited in concentric layers. The ideal pearl is perfectly round and smooth, but many other shapes of pearls exist. Nacre, also known as mother of pearl, is an organic-inorganic composite material of aragonite (a form of calcium carbonate) produced by some mollusks as an inner shell layer. Nacre is secreted by the epithelial cells of the mantle tissue of various mollusks. The layers of nacre smooth the shell surface and help defend the soft tissues against parasites and damaging debris by entombing them in successive layers of nacre. In a natural pearl, the irritant may be a grain of sand. In a cultured pearl, it might be a round bead. Nacre formation is mediated by the irritant, which controls the onset, duration and form of crystal growth. This growth forms either a blister pearl attached to the interior of the shell, or a free pearl within the mantle tissues. This process is called encystation and it continues as long as the mollusk lives. Nacre also makes up the hard, iridescent outside of the pearl. It is very strong, resilient. The nacre plates are only10–20 µm wide and 0.5 µm thick arranged in a continuous parallel lamina. Nacre appears iridescent because the thickness of the aragonite platelets resulting in different colors of light being reflected at different viewing angles. The finest pearls have been highly valued as gemstones. The most valuable pearls occur spontaneously in the wild, but they are extremely rare. These are called natural pearls. Cultured or farmed pearls from pearl oysters and freshwater mussels make up the majority of those that are currently sold. These mollusks can grow more than one pearl at a time yielding a higher production of pearls. Whether wild or cultured, pearls are graded on their size, shape, color, quality of surface, orient and luster. Pearls have been harvested and cultivated primarily for use in jewelry, but in the past they were also stitched onto lavish clothing, crushed and used in cosmetics, medicines, and in paint formulations. While museum quality pearls are very rare and expensive, pearls are more affordable and accessible than ever. Stay tuned. Next we will talk about how to pick out pearls you can enjoy and afford.