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Smithsonian National Museum of Natural History

Opal

Opal from Idaho, United States
Photo by Ken Larsen. Click to zoom.
Catalog Number NMNH G10520-00
Locality Idaho, United States

Gift of Bob and Susan Thompson in 2005.

The opal deposits in Spencer, Idaho, about 70 miles west of Yellowstone National Park, formed in a rhyolite-obsidian rock filled with gas pockets, or cavities. The opal, a form of silica, was hydrothermally deposited when geyser activity brought silica rich water to the earth’s surface over 4 million years ago. As this water seeped inside the hollow cavities, the opal formed in successive layers. The thin layers of opal that formed are extremely high quality with great transparency and a vivid play-of-color. Approximately 10% of the material is thick enough for cutting a solid opal cabochon; however, most of the material is too thin and therefore is fashioned into opal doublets and triplets. Opal doublets are thin slices of precious opal glued with epoxy onto a matrix material, usually black basalt. Opal doublets are often times covered with a thin rock crystal quartz cap to make them more resistant to scratches and breakage. This is known as an opal triplet. This spectacular opal triplet, named “Grand Finale” by the donors, measures approximately 51 x 83 x 8.5mm and exhibits vivid flashes of red, yellow, green and blue play-of-color resembling fireworks.


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