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How Pearls Form

Composition

Contrary to popular belief, pearls hardly ever result from the intrusion of a grain of sand into an oyster's shell. Instead, a pearl forms when an irritant such as a wayward food particle becomes trapped in the mollusk. The animal senses the object and coats it with layers of aragonite ("ah-RAG-uh-nite") and conchiolin ( "KON-kee-uh-lin"). These two materials are the same substances the animal uses to build its shell.

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Musee Oceanographique, Monaco OBJ-01335 97 6048.

Nautilus cup. Shell of Nautilus pompilius, silver. France, 1830


Structure

In most pearls, the mineral aragonite is arranged in sheets of flat, six-sided crystals. Between each sheet, the mollusk secretes a very thin layer of the membrane-forming protein conchiolin. This composite material is called nacre ("NAY-ker") or mother-of-pearl. The crystalline structure of nacre reflects light in a unique way, giving so-called nacreous pearls their high luster. In contrast, some pearls are not nacreous and instead have a low-luster, porcelainlike surface. The needlelike crystals of aragonite in these pearls are arranged perpendicularly or at an angle to the surface of the pearl.

Because of the soft nature of aragonite, the shell of this Chambered Nautilus could be easily carved. This cup features carvings of small cupids, or putti, a common motif of the 19th century.

American Museum of Natural History

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