fossil formed by the invasion of minerals into cavities between and within cells of natural wood, usually by silica (silicon dioxide, SiO₂) or calcite (calcium carbonate, CaCO₃). The petrified forests of the western United States are silicified wood, the tree tissues having been replaced by chalcedony (cryptocrystalline quartz). Often this replacement is so accurate that the internal structure as well as the external shape is faithfully represented; sometimes even the cell structure may be determined.