Pearl
When a bivalve mollusc (oysters, mussels...) enters an external particle that irritates them (mainly a grain of sand), to protect themselves, they wrap this particle in mother-of-pearl so that it stops irritating them.
Nacre is created through organic mediation, it consists of calcium carbonate in the form of aragonite crystals and conchiolin that serves as a matrix.
Most of the pearls on the market are cultured, although they can also be natural.