silicified wood

Silicified wood trunk

a sparkling, glittering part of a sequoia
a special feature from the Harz Mountains in Germany
Age: approx. 30 million years
Size approx. 28 x 5 x 5 cm 

in their lifetime, these sequoia trees have been overturned. In certain areas these tree trunks have gathered and sunk.
In the course of millions of years, lignite was formed from these trunks and under special conditions, sparkling, glittering quartz crystals were formed on them.

For all those who want to know exactly:

Silicification, also called silicification, is the impregnation or transformation of a naturally occurring mineral or organic material with or into microcrystalline silicon dioxide (SiO2). Silicification can be achieved by the precipitation of dissolved SiO2 in the pore space of sediments or soils or by metasomatic or
diagenetic displacement of other minerals by the SiO2 (dissolution of the previous mineral with simultaneous precipitation of SiO2)

The term silicification is derived from the word silica, which denotes various forms of SiO2 dissolved in water and is often used synonymously 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 milieu 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 caused by 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, which replaces the entire organic material.
In the further course of time the silica gel changes into opal-like aggregate states and finally into quartz.