Hillman Hall of Minerals and Gems

October: Opal or “Tourmaline”

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Opal is chemically like quartz but with water incorporated into its molecular structure. Opal is a product of low-temperature hydrothermal activity and is found in volcanic regions like Yellowstone National Park. It ranges from colorless, white, and gray to pastel blue, green, and pink. Only when it exhibits bright “fire” is it valued as a gemstone. The most valuable opal is black or dark blue-gray with brilliant red, blue, yellow, green, and orange fire.

Most precious opal comes from Australia, Brazil, and Mexico. The Virgin Valley in Nevada produces beautiful black opal but it is susceptible to “crazing”, or cracking, and is not often used in fine jewelry.

“Tourmaline” is the name for a group of minerals with similar chemical composition and molecular structure. Most tourmaline of the gemstone industry is the mineral elbaite, which occurs in most colors and color combinations, green being the most common. The finest tourmalines come from Brazil, Afghanistan, Pakistan, and the United States. Beautiful gemstones and crystals have been produced from Maine and California.

Opal and Fire Opal                             
Photo by Debra Wilson
  "Tourmaline" in various colors              
Photo by Debra Wilson
     
Opal   Tourmaline
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