Petrified Wood, wood that has been fossilized by being turned into stone. Petrifaction is a natural process that may form fossils from the solid remains of any plant or animal. However, it more commonly happens to wood than to animals and nonwoody plants. The process occurs when the substance soaks, over a long period of time, in water containing a mineral (or minerals). Gradually, the mineral either fills every pore and cavity of the organic matter, or the mineral-containing water dissolves the original organic material and replaces it with mineral matter. Eventually, the mineral forms a perfect copy of the original substance, including cell structure and fibers. The replacement mineral is usually a variety of quartz, ordinarily of the chalcedony type. Sometimes it is opal, calcite (calcium carbonate), or carnotite, a source of uranium.

Petrified wood usually is very hard. It may be beautifully colored by chemical impurities such as iron and copper. Cut and polished petrified wood is used for jewelry, paperweights, and lamp bases.

The rate of petrifaction is not exactly known. In some cases it may be fairly rapid. For example, mine timbers have been partly petrified after a few years' exposure to mineral-laden water. Most petrified wood was formed long ago. For instance, stone logs in Petrified Forest National Park, Arizona, are of the Triassic Period and more than 160,000,000 years old.

Petrified forests are found in various parts of the world. They were formed in different geologic periods and the trees in each one are the kind that grew during the period of its formation. Perhaps the oldest petrified forest in the United States is that of Devonian trees near Gilboa, New York. There is an abundance of petrified wood in Yellowstone National Park, including a few very rare upright fossilized tree trunks. Gingko Petrified Forest is a state park in Washington. Deposits of petrified wood are also found in California, New Mexico, and Utah.