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.
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.