How Do Pearls Form Inside of Clams/Mollusks/Oysters?

Natural pearls have been highly prized for millennia.

Among the first gems prized by humans, pearls are produced by mollusks in a variety of shapes, sizes and colors. Though oysters and clams are most known for making pearls, mussels, snails, conch and abalone also produce these gems. Pearls come from mollusks that live in warm water--whether it's fresh or salt--and can occur naturally or be cultured.

Result of a Defense Mechanism

Pearls are the result of a defensive mechanism by mollusks. When a foreign piece of material--be it food, a parasite, a piece of shell or sand--becomes lodged in a mollusk's delicate interior, it coats the object with substances called aragonite and conchiolin. Over time, these deposits build up around the material, creating a spherical pearl or other such gem.

How Coating Forms

Aragonite is a form of calcium carbonate, which is arranged in flat sheets of six-sided crystals. Between each of these aragonite sheets is a layer of protein forming a membrane, called conchiolin. Together, the aragonite and conchiolin produce what is commonly referred to as "mother of pearl" or " nacre," which is the lustrous coating on the inside of mollusks. Aragonite reflects light in a unique way due to its needle-like, crystalline structure that forms perpendicularly to the pearl's surface. This light reflection gives pearls their distinctive luster.

Natural vs. Cultured Pearls

Pearls occur naturally, but only rarely. Natural pearls are also usually irregular shaped and small; the mollusk is concerned only with covering the foreign object, not in making a perfect sphere. The size and shape of the object, as well as how evenly the mollusk coats the foreign material. determine the pearl shape. Many pearls seen today in jewelry are cultured, or produced through human manipulation. Cultured pearls occur when humans insert a foreign object into a mollusk (usually a pearl oyster) to force the mollusk to coat it with nacre. Over time, pearl farmers found that if they grafted a piece of mantle from the shell itself to a shell bead, then inserted that into the gonad of the oyster, a well-formed pearl would be produced.

Variations

Pearls vary widely in appearance, depending on how they are formed, from lustrous white, to duller porcelain white, to pink, to black to bronze. Abalone pearls, for instance, have a honeycomb-like structure to their crystals, producing an unusual surface sheen. The Black Pearl, on display at the Smithsonian Institute, is one of the finest black pearls known today, displaying a rare blend of bright colors and high sheen. The Hope Pearl, one of the largest natural pearls ever found at 450 carats (and also on display at the Smithsonian), changes shades gradually from bronze to white. It was probably a blister pearl. Blister pearls form when a foreign object becomes fixed only to the outer shell. The exposed portion, then, becomes covered in nacre, forming a blister pearl.

Signs of Wealth

Pearls have been a sign of wealth and privilege for thousands of years, precisely because the processes that must come together in just the right way at just the right time to produce an even, well-formed pearl in nature are extremely rare. Archaeologists found vaults filled with pearls at ancient Egyptian sites, indicating the wealth of the owners. Pearls have been used around the world as signs of opulence and beauty--from the South Pacific, to Asia to the Middle East. Even today, well-formed, large pearls--particularly natural pearls--are highly prized and highly priced for their beauty and rarity.

References

• Smithsonian Institute: Allure of Pearls
• American Museum of Natural History: What are Pearls?
• Fisheries and Aquaculture Department: Pearl Formation