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The pearl is the result of a natural reaction of the mollusk against external invaders, such as certain parasites that try to reproduce inside it. To do this, these organisms pierce the shell and lodge in the mantle, a thin layer of tissue that protects the oyster’s viscera. When defending itself from the intruder, it attacks it with a substance secreted by the mantle, called nacre or mother-of-pearl, composed of 90% of a calcareous material – aragonite (CaCO3) -, 6% of organic material (conqueolin, the main component of outer part of the shell) and 4% water. Deposited on the invader in concentric layers, this substance crystallizes quickly, isolating the danger and forming a small rigid acorn. The perfectly spherical pearls only form when the parasite is completely covered by the mantle, which causes the nacre secretion to be evenly distributed. “But the most common thing is for the pearl to stick to the shell, like a kind of wart. That’s why spherical ones are so valuable,” says biologist Luís Ricardo Simone, from the Museum of Zoology at the University of São Paulo (USP). The average maturation time for a pearl is three years.
As the oyster already defends itself very well against invaders with its shell, the phenomenon is rare, occurring in nature in only one in every 10,000 animals. At the beginning of the 20th century, the Japanese invented a simple way to speed up the process, introducing a small mother-of-pearl ball, taken from a shell, about three-quarters of the desired final size into the oyster.
The result is so good that, even for an expert, it is difficult to distinguish the natural pearl from the cultured one. Substances present in water can also be incorporated into the pearl, which is why its color varies according to the environment, generating the most diverse shades. The pearl is the only gem of animal origin.
Until the 17th century, there was no technology to polish precious stones such as rubies and emeralds, which is why pearls were one of the greatest symbols of wealth and power, used as adornments in the most valuable jewels of the time.
Pearl color varies depending on environmental conditions and the health of the oyster: the most common are pink, cream, white, gray and black
Pearl shapes depend on the shape of the invader and where it settles. The spherical ones are the rarest and, consequently, the most valuable.
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Organisms that invade the shell initiate the pearl formation process1. After piercing the shell, the invading parasite comes into contact with the mantle – the oyster’s defense tissue – and causes irritation inside the mollusk2. The mantle immediately goes into defensive reaction, folding over the parasite in such a way as to leave it completely isolated.
3. The defense is completed with the secretion of nacre, or mother-of-pearl, the same substance that produces the shell. It is deposited on the invader, forming a protective layer
4. Even after the source of discomfort is isolated, the pearl continues to grow, as the oyster does not stop secreting nacre