Petrified wood is also known as fossilised wood. It is formed when a tree or tree-like plant changes to stone by the process of permineralization. I have taken this paragraph from Wikipedia as it best explains the process: “Permineralization, a type of fossilization, involves deposits of minerals within the cells of organisms. Water from the ground, lakes, or oceans seeps into the pores of organic tissue and forms a crystal cast with deposited minerals. Crystals begin to form
