A friend gave me this big chunk of petrified wood some time ago so that I could add it to my collection of minerals. As I remember it he got it on a family vacation in Arizona where there was loads of petrified wood lying around at the back of some gas-station. He packed his back pack full but had to hand over most of it at the border because of some legal restrictions. I might have misremembered most of the story, but the origin of this chunk is probably accurate as Arizona is filled with sites where there is vast petrified forests.
A piece of petrified wood is a sort of fossil that forms when the wood gets buried under sediment for hundreds of years. Encapsulation of the wood in the protective and oxygen-free environment of the sediment, enables mineral-rich water to slowly replace decaying organic cells one by one with everlasting crystals in various colors. This process preserves the structure of the wood in such a way that one can even make out the year rings of individual logs.
This image is from the Petrified Forest National Park in Arizona