Petrified wood is actually fossilized wood. According to gemologists, there is no wood left in petrified wood, only rock that takes the same form and shape of the wood. Normally, trees decompose, but when a tree lands in a swamp or is otherwise covered up, decomposition is halted. The cells within the tree are hollowed out, and mineral-rich water seeps in. As the water evaporates, the minerals are left behind, creating a copy of what was there. 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.
So here it is - the oldest piece of whimsy in my garden!