See Pearls in a Whole New Light

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A Brief History on Pearls

A pearl forms when something gets trapped inside a mollusk. The mollusk senses the object and proceeds to coat it with layers of the same substances it uses to create it’s shell: aragonite and conchiolin. In most pearls, aragonite is arranged in flat sheets of 6-sided crystals. Between each sheet of aragonite is a thin layer of the membrane-forming protein conchiolin. This composite material is called nacre or mother-of-pearl. The crystalline structure of nacre reflects light in a unique way, giving the pearls their high luster. In contrast, some pearls are not nacreous and instead have a low-luster, porcelain like surface. The needle like crystals of aragonite in these pearls are arranged perpendicularly or at an angle to the surface of the pearl.

Pearls occur naturally in a spectacular array of colors. A pearl’s color depends on both the species of mollusk that produced the pearl and the environment in which the animal lived. In general, crystals of aragonite are white or colorless. The natural color of a pearl is mostly due to conchiolin, which contains organic pigments.

One of the most distinctive features of a nacreous pearl is the way it seems to glow from within. This property, known as “luster,” gives pearls their unusual beauty. Luster results from the reflection of light rays not only off the surface of the pearl, but also off the concentric inner layers of nacre. Because a pearl’s surface is round, it acts as a convex mirror, reflecting light so that it appears to emanate from within the pearl. The multiple layers of nacre also give rise to the “iridescence” or “orient” of pearls–a characteristic that resembles the shimmer seen on a soap-bubble. The layers of nacre act like tiny prisms, refracting light so that it appears as all the colors of the rainbow.

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*Information on pearls obtained by American Museum of Natural History

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