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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.

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.

im_form_nautcup

Nautilus cup
Shell of Nautilus pompilius, silver
France, 1830

Musée Océanographique, Monaco OBJ-01335 97 6048.


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.

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