Hard and brilliant concretion composed of concentric layers of mother-of-pearl as produced by the oyster mantle epithelium to protect the oyster from the inclusion of a foreign body, as initiated in this case by human intervention as part of pearl oyster culture. This culture takes place in warm seas, along the shores of Myanmar, Japan, Australia or even Tahiti. This Polynesian archipelago produces magnificent black pearls.

Some cultured pearls are also produced in fresh water using a variety of mussels (pearl mussels). Fresh water cultures are found in Scotland, Sweden, on the shores of Russian lakes, in France (the Vosges and Charente areas),, in the US (Mississippi) or even in China.