Petrified Wood

Petrified Wood is wood chemically replaced by a mineral substance. The replacement is usually 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. In the 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 silicafied logs that are found in the area.
When the mineral replacement of the wood is Chalcedony or Opal, the substance is specifically called "Silicafied Wood".