How does wood turn to stone? Petrified wood forms after a tree trunk becomes covered by wet mud composed of fine sediments or volcanic ash. Water keeps the wood from decaying quickly, and the mud provides silica that gradually replaces the wood’s cells with quartz, opal, or chalcedony (all are forms of silica). Sometimes this happens so slowly that the tree’s rings and cells are preserved. Some petrified wood can be very colorful because iron and manganese impurities can tint the silica red, yel