Petrified Wood

The U.S. National Park Service describes the process at Petrified Forest National Park in Arizona, USA: over 200 million years ago logs washed into a river system were buried quickly and deeply enough by sediment to cut off oxygen and slow decay; silica dissolved from volcanic ash was then absorbed into the porous wood over hundreds to thousands of years and crystallized within the cell structure, replacing the organic material as it broke down. Groundwater in areas with volcanic ash is especially rich in silica, and the park's petrified logs lie in the Upper Triassic Chinle Formation; most of the trees there are named Araucarioxylon arizonicum, and clear quartz, amethyst, citrine and smoky quartz crystals grew in cracks in some logs.

Petrified wood keeps the original three-dimensional shape of the wood, sometimes down to microscopic or cellular detail, and it is denser and heavier than the original wood because its pores are filled with silica or other minerals. Its quartz is hard and brittle and breaks with clean fractures. Not all fossil wood is petrified: some has been carbonized instead.