It takes millions of years for wood to become petrified. According to isotopic geological data reported by the National Park Service, trees in the Petrified Forest National Park in Arizona are between 211 and 218 million years old.
Petrified wood results when felled trees become buried under mud or volcanic ash. The wood, sealed off from oxygen, interacts with mineral compounds, such as silica. Over millions of years the silica crystallizes within the cellular structure of the wood, at which point it is petrified. Petrified wood often takes on the colors of the minerals that have solidified in its pores.