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Smithsonian National Museum of Natural History

Allure of Pearls suite

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Photo by NMNH Photo Services. Click to zoom.

What is a pearl?

A pearl is mostly aragonite crystals, a form of calcium carbonate (CaCO3) which also makes up other marine organisms from coral to sea urchins. Pearls are produced by a variety of mollusks in warm fresh and sea waters around the world. They appear in a wide range of colors, shapes, and sizes.

How do pearls form?

The mollusk protects its delicate interior by coating foreign objects (usually a tiny piece of shell or parasite) with calcium carbonate, the same substance that lines the inside of their shells. This “seed” grows larger as the mollusk continues to coat it. Among the mollusks that can produce pearls are mussels, oysters, clams, snails, conch, and abalone.

How are pearls judged?

Similar to other gems, pearls are judged by their size, weight, shape, surface quality, thickness of the nacre (the coating produced by the mollusk), color, orient (play of colors across the surface) and luster. Generally, the larger the pearl, the rarer and more valuable it is.

How do natural and cultured pearls differ?

Natural pearls occur without any intervention by humans and are found in fresh or sea waters. They tend to form more organic shapes and less than perfect spheres. Cultured pearls are the result of humans inserting foreign particles inside the mollusk. They grow in ocean or freshwater farms and generally are more uniformly colored and shaped.


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