Wandering Lizard
The San Francisco Bay Area

A nearby volcano erupted over three million years ago. The blast from the eruption and an associated earthquake toppled all of the redwood trees in this forest. Subsequently the trees were covered in volcanic ash. Over the millions of years that followed these events water seeped down through the ash to the decaying trees. The water leached silicates from the ash and transported it into the cells of the wood replacing wood with silicate - cell by cell. The process replicated the wood so acurately that if you look closely you can see the design of the wood and the bark.