Petrified Wood fossil formed by the invasion of minerals into cavities between and within cells of natural wood, usually by silica (silicon dioxide, SiO2) or calcite (calcium carbonate, CaCO3). Often this replacement is so accurate that the internal structure as well as the external shape is faithfully represented; sometimes even the cell structure may be determined. Petrified Wood is such a wood that is chemically replaced by a mineral substance. The replacement is usually done by Chalcedony ('silica'), but Opal and other minerals are also known to replace the wood. When the wood becomes petrified, its original mold remains intact, but an entire new substance takes the place of what was once wood.
Petrified woods are important fossils that can give ideas about the environment under which they were formed no matter what sort of silica replaced its original chemical properties. Petrified Forest National Monument in Arizona, an entire forest was transformed into petrified wood. Remains of this ancient forest can be seen in the huge silicified logs that are found in the area. When the mineral replacement of the wood is Chalcedony or Opal, the substance is specifically called 'Silicified Wood'.
In Bangladesh petrified woods are found in some places particularly in the barind tract as well as in the madhupur tract areas. This is a characteristic marker for the Dupi Tila sediments of Bangladesh. It is found abundantly in the Dupi Tila sediments of the Lalmai-Mainamati areas. Petrified woods in the form of leaf impressions are also found in a small island in kaptai lake. Locally known as Asurer Haddi (Bone of Giants) and used by the indigenous medicine-man for healing different diseases. [Kazi Matinuddin Ahmed]