The formation of a pearl, also called margarita, occurs due to the penetration of substances, particles (sand) or microorganisms (worms) between the shell and mantle of some oyster species.
In response to the foreign body, the mantle secretes a series of layers of nacre or mother-of-pearl, a substance composed of calcium carbonate pectates in the form of aragonite crystals, playing a defense mechanism of the body. Naturally, this process gives shine to the shell surface, and likewise to the pearl, which does not need cutting or polishing.
Cultured pearls are produced by inserting, artificially, into other pearl producers, a small sphere surrounded by a fragment of the mantle extracted from a young oyster.
By applying this technique, a pearl takes an average of three years to form. The Japanese are the pioneers in the cultivation of these molluscs, the oysters of the species: Pinctada imbricata, Pinctada maxima and the Pinctada margaritifera, produce cream colored pearls, yellowish greenish or black, with diameter ranging from 2 to 17 mm.
By Krukemberghe Fonseca
Graduated in Biology
Source: Brazil School - https://brasilescola.uol.com.br/biologia/a-formacao-uma-perola.htm