Petrified wood is a type of fossil created by the millennia-long process of permineralization. It begins with a tree that is buried quickly, perhaps by volcanic ash or mud. In these oxygen-free conditions, the tree is protected from decay. Groundwater, rich in dissolved minerals like silica, seeps into the wood and fills the tree’s cells and pores. Gradually, they replace the original organic material, molecule by molecule, preserving the microscopic structure of the wood.