The lack of hard parts in soft-bodied organisms makes them extremely rare in the fossil record. It takes very special conditions for them to be preserved where they are buried rapidly in areas devoid of oxygen and scavengers.
Petrified wood is a type of fossil. It forms when organic wood is rapidly buried in sediments that protect it from decay. Groundwater rich in dissolved minerals then replaces the original plant material with minerals such as silica (quartz) in a process called permineralization.