In Greek mythology, merely glancing at the ugly head of Medusa could turn the looker into stone. For wood, the process is not so fast.
Petrified wood forms when fallen trees get washed down a river and buried under layers of mud, ash from volcanoes and other materials. Sealed beneath this muck deprives the rotting wood from oxygen — the necessary ingredient for decay. As the wood's organic tissues slowly break down, the resulting voids in the tree are filled with minerals such as silica — the stuff of rocks.
Over millions of years, these minerals crystallize within the wood's cellular structure forming the stone-like material known as petrified wood. The wood, no longer wood at this point, takes on the hues of the minerals that fill its pores. Minerals such as copper, cobalt, and chromium give off a green-blue color, while manganese presents a pink hue.