Because of the abundance and popularity of this item, the Washington State Legislature designated Petrified Wood as the state gemstone in 1975 (though rock-hard and beautiful, petrified wood is not actually a gem but a fossil).
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The most popular and abundant fossil to garner state honors is Petrified Wood. Most of the petrified wood in Washington grew during the Miocene Epoch, some 5 -12 million years ago, when the state was swampy and mild, and played host to vast forests of cypress, oak, elm, and ginkgo trees. Petrified wood forms when logs are buried, and their organic matter is replaced by minerals in the groundwater, primarily silica. Although much petrified wood is buried in river sediments and is thus found in mudstone or sandstone, the trees in ancient Washington grew next to large volcanoes which spewed tons of ash into the air when they erupted. This volcanic ash settled and buried the trees in place; sometimes they were even engulfed by lava flows. The major petrified wood-bearing unit in Washington is the Columbia Plateau basalts. The most famous petrified wood site in the state is Ginkgo Petrified Forest State Park near Vantage. Many types of wood are preserved there, but the abundance of Ginkgo wood gave the park its name. Ginkgos are gymnosperm trees (non-flowering plants in the same grouping as pines, spruces).
.Although Petrified Wood occurs in the western portion of the state, it is rarer than in the east. The western portion of North America is a newcomer by geological standards, consisting of a mèlange of terranes which have been splatted up against the ancient core of the continent by plate tectonic action. Thus the eastern portion of the state has been dry land for a longer period of time, and has had more time than coastal regions to develop forests and fossils.
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Petrified wood of different varieties are also the state stone of Texas, and the state fossil of North Dakota and Louisiana.
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