Fossil Opalized Petrified Wood Limb Section

Name: Plantae, sp. indet.

Geological Time: Miocene, Luisian Stage

Size (25.4 mm = 1 inch): 100 mm long by up to 30 mm thick; weight 87 grams

Location: Virgin Valley, Humboldt County, Nevada

Opalized Petrified WoodDescription: This specimen of highly-colored petrified wood from the Luisian Stage, (~14 million year old) deposits of The Virgin Valley region of northwest Nevada is a limbcast preserved as precious opal. Some 16 million years ago the Virgin Valley was formed during a series of rhyolite volcanic flows, resulting in a large basin enclosed by low hills. This basin contained a succession of lakes and forests of spruce, hemlock, birch, chestnut and even sequoia which were periodically buried by volcanic ash hundreds of feet thick. A large lake formed within the basin which deposited large amounts of diatomite, a biogenic form of silica. Seepage of super-heated water percolated through the ash layers, carrying silica to the long-buried trees.

Replacement of carbon in the wood by hydrated silica resulted in perfect opalized replicas of the original wood structure. It is the alignment of the hydrated silica spheres which ultimately results in the rainbow effect of precious opal, the result of deflection and diffraction of light as it passes through the planes of hydrated silica molecules. The size of the spheres impacts the colors seen, with smaller spheres resulting in blues and larger spheres in reds. While common opal is abundant in the region, the conditions required for formation of precious opal as seen here was far more rare, a combination of a stable and undisturbed environment. While much of the world precious opal is found in Austarlia, deposits such as those in the Virgin Valley are also mined for these treasures of a bygone world.

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