Petrified Wood
Millions of years ago, a fallen or dead tree was buried in sediments, which included a considerable amount of volcanic ash. Silica found in the ash got dissolved by ground water, and percolated down to the buried wood. The silica filled or replaced cell walls, crystallizing as chalcedony. Other minerals, also present in the sediment, give petrified wood a wide variety of color from grey and brown to bright colors such as red and green. Famous deposits of petrified wood in the USA can be found in Oregon, Washington, Wyoming, and Arizona. Fine specimens are also found in Argentina and Africa.
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