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