Petrified wood is formed when water containing dissolved minerals such as calcium carbonate (CaCO3) and silicate infiltrates wood or other structures. The process takes thousands of years. The foreign material either replaces or encloses the organic matter and often retains all of the structural details of the original plant material. Botanists find these types of fossils to be very important since they allow for the study of the internal structure of extinct plants. After a time, wood seems to have turned to stone because the original form and structure are retained. The wood itself does not turn to stone.