Hillman Hall of Minerals and Gems

June: Pearl, Moonstone, or Alexandrite

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Pearls are the product of irritated shellfish. Each pearl begins as a tiny grain of sand or other material trapped within the shell of the animal. The irritant becomes coated by layer upon layer of the oyster's natural protective substance, nacre, composed of the mineral aragonite, as the hapless crustacean tries to reduce the irritation caused by the grain's presence. Today, oysters are farmed and seeded with artificial irritants composed of bits of shell or plastic beads to ensure a large supply of quality pearls. Pearls produced in this manner are known as cultured pearls.

Moonstone is a variety of the feldspar mineral albite that displays a flash of color, a characteristic known as chatoyance, when viewed in proper lighting. This effect is the result of crystal twinning on a microscopic scale and is common in the plagioclase feldspars. The color of the flash is usually blue-white with tinges of red, yellow, orange, or green, while the color of the stone itself can range from white, the most popular, to gray, brown, or orange.

Alexandrite, named for former Czar of Russia Alexander I, is a special variety of the mineral chrysoberyl. It exhibits a color change from green to red depending upon the type of light in which it is viewed. Historically produced from the Ural Mountains in Russia, Brazil currently produces fine gems of this type.

Albite variety Moonstone                             
Photo by Debra Wilson
  Melo Pearl, Vietnam
Photo by Debra Wilson
     
Albite   Beryl
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