Petrified wood: what is it? In essence, it is a fossil. Petrification is the process through which a tree or plant that resembles a tree turns into stone through mineralization. It happens when plant matter is buried by silt or volcanic ash and is shielded from decomposition by organisms and oxygen. The original plant material is then replaced by silica, calcite, pyrite, or … [Read more...] about In Arizona, There is a Petrified Opal Tree Trunk (About 225 Million Years Old)