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What Is Fossil Wood?

You may have heard of the Petrified Forest in Arizona and wondered what the name means. Petrified wood is fossilized wood that has become stone-like over time. This is unusual because most fossils are not formed from organic material, like wood and plants, they are commonly formed from bones, like dinosaur teeth.

Origin
- Petrified wood typically dates back millions of years, to the Cretaceous period (spanning 65 million to 145 million years ago) when dinosaurs went extinct, and as far back as the Triassic period (about 199 million years ago).
Conditions
- Typically, the wood was buried in wet sediment, with plenty of minerals but little oxygen or bacteria (which contribute to rotting). Frequently, these sediments contained carbon-rich volcanic ash. Carbon is a contributor to the colors in petrified wood and volcanic ash is a source of silica, one of the minerals that contributes to the fossilization process.
Structure
- There is no wood left in what we call petrified wood. Over time, minerals crystallize within the cellular structure of the wood, leaving hard quartz in place of the organic material.
Significance
- Fossilization of soft material like flesh, wood and plants, is rare.
  
  Organic fossils, formed from flesh and plants, are rare. What appear to be fossils of plants are really only the impressions of the plants--the actual plant has rotted away. Petrified wood is possible because a dense tree can survive the fossilization process in a way that a tender, leafy plant cannot.
Location
- The richest deposits of fossilized wood are located in the United States, specifically in Arizona (home of the Petrified Forest National Park), California, Washington State and Colorado. Some deposits exist in Africa and China, but not to the extent found in the U.S., where petrified wood is plentiful enough to be sold as souvenirs.

Gemology