From Academic Kids
Petrified wood is a type of fossil, in which the tissues of a dead plant are replaced with minerals (most often a silicate, such as quartz). The petrifaction process occurs underground, when wood or woody materials suddenly become buried under sediment. Mineral-rich water flowing through the sediment deposits minerals in the plant's cells and as the plant's lignin and cellulose decays away, a stone cast is left in its place.
Petrified wood can be extremely detailed, often reflecting the internal structures of the plant from which they form. Structures such as tree rings and vascular tissues such as xylem are often observed features.
Petrified wood has a Mohs hardness level of 7, the same as quartz. Materials scientists have been able to make artificial petrified wood recently. (http://www.usatoday.com/tech/science/discoveries/2005-01-25-petrified_x.htm?csp=15)
A famous petrified wood site is Petrified Forest National Park in the U.S. state of Arizona. Wood from the area (though not from the park) is a prized commercial item, particularly cut and polished samples.
- The Petrified forest of Lesvos - Protected Natural Monument (http://www.aegean.gr/Petrified_Forest/NoFrames/petriforest.htm)