Fossil wood, aka petrified wood, or "Wu-mu" and "Yin-chen mu" in Chinese, is a fossil of a woody plant preserved by permineralization through time by chemical and physical processes.

The process of petrification is not completely understood because researchers have not been able to duplicate the process in the laboratory, where it can be observed and measured. But, certain conditions that must have existed for petrification to occur are known. Oxygen, which causes oxidation or rotting of all types of materials, would have to have been kept away from the dead plant material to prevent it from decaying before it was preserved. Most likely, the dead plant material was deprived of oxygen by being buried by sediments settling in water covering the plants. Much of the fossil wood found today is a product of ancient river and flood plain environments.

After rapid burial, the tree reacts to percolating water. Three things may happen. The log may disintegrate and not be fossilized. The log may be reduced by compression to a coal or it may become petrified. If petrification takes place, minerals from percolating water are deposited in fluid-filled openings in the wood. This process is called permineralization and it preserves the tissues of the wood! In some situations minerals may also substitute for the woody tissues of the log. This process is called replacement. Most petrified woods are permineralized . The entire process is not fully understood but is being actively studied.

The final condition, necessary for petrification, is time. The mineral replacement process is very slow, probably taking millions of years.

The original plants of fossil wood from Sichuan, China, include willow tree, toon, camphor tree and Phoebe zhennan etc. Because of the variety of the plants, the fossil wood present different colors such as blacjm dark brown, red or goldenrod. Scientists determined that their ages are approximately 6,000 to 40,000 years through uses of C14 isotope. It has been considered as precious material for furniture and artwork.

In Chinese, both ebony and fossil wood are called "wu-mu" although they are completely different in nature.