Petrified wood. Fossilised wood from a conifer (Agathoxylon sp.). Petrification is a term used to describe the fossilisation of wood. It occurs when pieces of wood fall into sediments and are rapidly covered, preventing normal decay. Over long periods of time the wood decays. As it decays, the wood is replaced by minerals which are deposited from solution in ground water. This forms an exact replica of the original, preserving the texture of the bark and even the growth rings.