Petrified or silicified wood

Wood fossilized Silicified

Silicification is the impregnation or conversion of a naturally occurring mineral or organic material with or into microcrystalline silicon dioxide (SiO2). The silicification can take place through the precipitation of dissolved SiO2 in the pore space of sediments or soils or through metasomatic or
diagenetic displacement of other minerals by SiO2 (dissolution of the previous mineral with simultaneous precipitation of SiO2).

The term silicification is derived from the word silicic acid, which denotes various forms of SiO2 dissolved in water and is often used as a synonym for the microcrystalline form of this chemical compound. In order for the process to take place, an aqueous solution containing silica in large quantities is first required.
This solution must also be able to circulate freely in the fissure or pore space so that it can reach areas with suitable chemical conditions for precipitation.

These particles are dissolved in the alkaline environment generally found in carbonatic sediments and the solutions circulate in the pore space of the sediment. 
The acidic environment is often created by bacteria that decompose organic matter.
Plant and animal remains can then silicify there.

Silicified wood is usually the result of silicification processes that begin after the wood is covered with thick layers of sediment.
In the course of time, first the cell interstices and cavities and finally the cell walls are filled with silica gel that replaces all organic material.
In the course of time, the silica gel changes into opal-like aggregate states and finally into quartz.