Petrified wood is actually wood that has turned into stone, as the organic material is replaced with minerals, usually quartz. So don't be fooled! No matter how much like tree branches these seem, they're really rocks! The original plant material is buried underground and can't get oxygen, necessary for decay. Dampness brings minerals to it, and the minerals gradually replace the organic material. Then, when the pieces make their way back to the surface, they're really rocks that look like bits of plants.