Description
Petrified wood forms when plant material is buried by sediment and protected from decay by oxygen and organisms. Groundwater, rich in dissolved solids, then flows through the sediment replacing the original plant material with silica, calcite, pyrite or another inorganic material such as opal. The result is a fossil of the original woody material that often exhibits preserved details of the bark, wood and cellular structures.
Some specimens of petrified wood are so accurate that people do not realize they are fossils until they pick them up and are shocked by their weight. These specimens with near perfect preservation are unusual however specimens that exhibit clearly recognizable bark and woody structures are very common.
Source: https://wyrdscience.wordpress.com/2011/01/02/fossil-forests-reveal-a-subtropical-antarctica/