Petrifaction

In geology, petrifaction, petrification or silicification is the process by which organic material is converted into stone by impregnation with silica. It is a rare form of fossilization. Petrified wood is the most well known result of this process, but all organisms from bacteria to vertebrates can be petrified.

Process
A major ingredient in the cell walls of wood is cellulose. Under certain conditions, the silica binds to the cellulose; when it crystallizes, it preserves the microscopic structure of wood even after the cellulose is gone.

Petrification is not the same as permineralization. Permineralized animal or vegetable remains have their original pore space infilled with minerals. In petrification, while the pore space is infilled with minerals, the original material of the organism is also replaced with minerals.