Fossils are organic matter - plants or animals - whose original material has been completely replaced at a molecular level by minerals. Petrified wood is wood turned to mineral or stone and although it looks like wood - it has become rock.

In order for fossilization to occur the material must have been buried by water or volcanic ash in such a way that no oxygen could reach it. Minerals in the water or ash slowly replace the organic materials with minerals like calcite, pyrite, and silica.

Fossil wood like that shown in the picture to the left is a product of ancient river and flood plain environments.