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How Do Pearls Form?

Do pearls form around grains of sand trapped in oysters? Learn the facts about pearl formation and the various mollusks that create these organic gems.

7 Minute Read

What Are Pearls Made Of?

Before discussing pearl formation, let’s talk about nacre. Nacre is the term for the conchiolin and layered plates of aragonite that form pearls and mother of pearl.

Aragonite is a form of calcium carbonate, the same compound that makes calcite. Conchiolin is a specialized organic substance. It helps the mollusk precipitate aragonite from the surrounding waters and acts as a glue, holding the layers together.

Nacre gives a pearl its unique, iridescent sheen. Aragonite plates range in thickness from about 300 to 1500 nm, similar to a wavelength of light. Therefore, they can cause diffraction, which appears as iridescence when reflected back to our eye.

However, the thicker aragonite plates are too thick for diffraction of visible light. When a pearl has thick aragonite in its outer layers, it appears chalky instead of iridescent.

How Do Natural Pearls Form?

Natural pearls form without any human interference. For most of human history, this was the only way that a pearl could form. It was very rare, though, to find a natural pearl of any quality. To get a matching strand, years of pearl harvests would be carefully sorted to find pearls that, by today’s…

Addison Rice

A geologist, environmental engineer and Caltech graduate, Addison’s interest in the mesmerizing and beautiful results of earth’s geological processes began in her elementary school’s environmental club. When she isn’t writing about gems and minerals, Addison spends winters studying ancient climates in Iceland and summers hiking the Colorado Rockies.

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