How do bivalves produce pearls?

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Most people know that pearls come from oysters. But not everyone knows that pearls do not usually start as grains of sand, that oysters are not the only mollusks to produce pearls and that pearls are the only gems not created deep inside the earth, but rather are created inside a living animal.

The shells of mollusks and the pearls they produce are biominerals. They are made of calcium carbonate, as are chalk, egg shells, limestone, and marble. What is the secret ingredient in a pearl that transforms this common mineral into a jewel? How do lowly bivalves produce such lovely works of art that have lived in the imagery and folklore of cultures throughout the world and throughout history? How do they create the jewel that has symbolized richness, elegance, purity, virginity, nobility, the moon, knowledge, and truth and is used to describe the very gates of Heaven?

To understand the process of pearl creation, it is important to understand the anatomy of a bivalve. Although a pearl can occasionally be produced by a snail, it is the bivalve that creates the unusual beauty that civilizations have cherished through the centuries. Clams and mussels also can produce pearls, but of the bivalves, oysters are the most productive.

As the name implies, the bivalve is a mollusk that has two shells. The shells are called valves because they regulate the flow of water through the gill-like structures of the soft-bodied invertebrate creature inside. The shells are hinged together by a strong ligament.

Mollusk is another name for shellfish, although not all mollusks have shells (for example, squid and octopus), and they are not fish. Many live in water, but there are those, like land snails (which are univalves) and slugs (which also do not have shells) that do not. The evolution of the mollusk began about 530 million years ago. Today there are about 100,000 different species of mollusk.

If a mollusk can produce a shell, it can produce a pearl, but it rarely does. Perhaps one in 10,000 mollusks actually produces a pearl in its natural habitat. The cultured pearl industry, begun in the early 1900s, has developed techniques to induce production.

It is commonly known that pearls start out as irritants that get trapped between the two valves. However, the irritant is not usually a grain of sand, but rather a small particle of food. The secret ingredient that performs the bivalve’s magic is nacre, a combination of layers of the same secretions from the bivalve’s mantle, aragonite and conchiolin, that the animal uses to create its shell. Aragonite is a crystalline form of calcium carbonate. Conchiolin is a protein substance used to hold the layers together.

Sensing the irritant that is trapped inside, the animal coats the particle with alternate layers of each of these secretions, the combination resulting in nacre, or mother-of-pearl. Nacre can also form the inner layer of a shell. When it surrounds the irritating particle, it forms a pearl.

The iridescence of a pearl is due to the flat arrangement of crystals in the aragonite layer that refract the light in the unique way that gives the pearl its shimmering quality. Pearls that are more like porcelain have a perpendicular or an angular arrangement of crystals in the argonite layer and do not refract light in the same lustrous way.

Perfectly round pearls are very rare in nature. The layers that the bivalve secretes are not always even and contain surface irregularities. Pearls also come in many different colors, depending on the species of bivalve and the composition of the water. The argonite layer, which contains organic pigments, gives the pearl its color.

While pearls of different colors can be found around the world, black pearls seem to be exclusive to the South Pacific.

Resource by Will

I am the editor for TheFreeResource.com, and owner of LinchpinSEO, a small business content creation company. The Free Resource has a select community of talented writers, teachers, and business people (and we are always looking for more) that bring you the most trusted resources on the web and credible articles about many topics.

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