what are pearls made of

Pearls are organic mineral deposits that have been valued as precious gemstones throughout human history. They are formed inside mollusks, such as oysters and mussels, as a natural defense against an irritant such as a parasite entering their shell or damage to their fragile body. The mollusk slowly secretes layers of aragonite and conchiolin, materials that also make up its shell, which creates a material called nacre, also known as mother-of-pearl, that encases the irritant and protects the mollusk from it. Over time, the layers of nacre build up, forming a pearl inside the mollusks shell. The ideal pearl is perfectly round and smooth, but many other shapes, known as baroque pearls, can occur. Pearls are composed of calcium carbonate, mainly aragonite or a mixture of aragonite and calcite, in minute crystalline form, which has deposited in concentric layers. The most valuable pearls have a metallic, highly reflective luster.