First, let’s debunk a few widely held misconceptions, shell we?
A common myth: pearls form from a grain of sand that has found its way into the body of a mollusk.
Many centuries ago, someone claimed that a natural pearl forms when a foreign body — a grain of sand — invades the soft tissues of a mollusk, which then gradually coats the invader in nacre to neutralize the irritant. This explanation has been repeated so often that it is accepted as truth.
Both common sense and scientific analysis show that it is incorrect.
Common sense: if the mollusk’s goal in coating the foreign body with nacre was to neutralize the irritation, why does it continue layer upon layer, long after all traces of the invader have disappeared beneath smooth, native layers of mother-of-pearl?
The science: a foreign body enters the mollusk and either settles in the mantle tissue — where the epithelial cells that produce nacre are located — or picks up epithelial cells along the way before settling in the soft body. Whether in the mantle or elsewhere, those epithelial cells continue doing what is genetically programmed in them: producing nacre. Instead of continuing to grow the shell, they form a pearl sac and create a pearl that encloses the invader.
A cultured pearl with a bead nucleus is grown in exactly this way. Pearls do not form around grains of sand. They form around organic matter — a parasite, a fragment of tissue — and as a result of damage or displacement of epithelial cells in the mantle.
Akoya pearl nucleation operation on a pearl farm in Japan
The second point worth clarifying is the term “cultured.”
Although Akoya cultured saltwater pearls were the first to be farmed, they are not the only type that is cultured. All commercially produced pearls today — both saltwater and freshwater — are cultured. The word “cultured” in an advertisement adds nothing, because it describes virtually everything on the market. What must be stated explicitly, if it applies, is the word natural.
Cultured pearls divide into two major categories: pearls grown in saltwater and pearls grown in freshwater. Both are widely used in jewelry. Saltwater-grown pearls are generally considered rarer and more valuable.
Oyster
Although the term “pearl oyster” is used at all levels of the pearl industry, the most valuable pearls do not technically come from oysters. Real oysters belong to the Ostreidae family. Pearl-bearing marine mollusks belong to the Pteriidae family. Both are bivalves of the Mollusca type, but only Ostreidae are true oysters. In freshwater, pearls are cultivated in mussels from the Unionidae family.