Petrified wood forms much in the same way animal fossils form; decaying plant matter is replaced by mineral deposits. First, mineral-rich water leaves deposits in the pores of the wood, creating a rocky surface layer over the plant fiber. Then, the original wood decays, with more inorganic material filling in as it disappears. Eventually, the wood is completely replaced by stone, creating a petrified fossil.
Like animal fossils, petrified wood requires certain environmental conditions to form. The wood must be covered over by sediment fairly quickly to prevent it from simply rotting away, and it needs to be exposed to water with a very high mineral content to trigger the mineralization process. Finally, the oxygen content of the sediment must be low enough to preserve the wood as long as possible, creating the mould for the minerals to take over.
The appearance of petrified wood varies depending on which minerals were present during the petrifaction process. Quartz is a common material found in petrified wood, while the presence of iron oxide can give it a reddish hue and copper can turn it a vivid green. A single specimen of petrified wood may contain many different minerals, creating a rainbow effect over its surface.
The process of permineralization of wood takes a few years but lasts millions. The beauty of the color and intrinsic quality of petrified wood make it valuable in jewelry, paperweights, and other decor. It can be cut and polished or left in its natural state. There are forests of petrified wood in various parts of the world. Petrified Forest National Park in Arizona has stone logs that are more than 160 million years old.