03/18/2026
I recently went camping out in Woodruff, AZ and found some very nice Rainbow Petrified Wood and it got me thinking on how it's made. I wanted to share what I learned, with anyone who reads this and is interested.
A lot of the members at the lapidary lab purchase rocks, or slabs. I personally find it more gratifying to go out and find the rocks I work on. I also have small children and they love to go out with me, making memories, eating smores.
So we drove almost 4 hours to collect rocks, and here is what I learned.
Rainbow Petrified Wood is.. not wood.
and it's not petrified. 🤷♀️
Well, I guess it SORT of is petrified... but a better term to describe the process is Permineralization.
"Permineralization is the infilling of natural pores in original organic material by minerals." https://www.nps.gov/articles/000/permineralization-and-replacement.htm
Here is where I tell you why Rainbow Petrified Wood is not technically wood.
Well, about 200 million years ago, a bunch of trees got washed/knocked down into river beds and such and they were quickly covered by ash, sand, other rocks, dirt, debris, etc. They became so compacted underneath these layers that oxygen simply couldn't reach them. What happens when oxygen doesn't reach an organic material? The organic material takes a REALLY long time to decompose. Over time, groundwater full of minerals slowly trickled into the spaces where these trees were. The trees/tree pieces were still decaying, just at a much slower rate. This created the perfect storm of permineralization or replacement where the minerals in the groundwater slowly replaced the tree/tree pieces. Without the trees being in the spaces, it's likely some form of "rock" would have been created regardless, but it would not have the same structure or shape as what Rainbow Petrified Wood has. Even still, when I found a lot of the "rocks" that I did, a majority of them were small broken pieces or did not show a bark like shape on them at all. My theory for this is that some of the trees were pulverized into powders but still ended up compressed and were replaced by the minerals in the groundwater. In case at this point it isn't clear, the wood decayed slowly, but it DID decay, so the entire structure of the wood was completely replaced by minerals thus creating a FOSSIL.
That's right.. Rainbow Petrified Wood is actually a fossil. It's known as a replacement fossil and it's a similar process to what happens to dinosaurs. I think technically it could be considered both a replacement fossil and a mineral or rather a collection of minerals, similar to other rocks we know of.
Final bit of information, some of the main minerals that percolated into the spaces that the trees, tree pieces, bark, powders, etc were in are iron, carbon, and manganese and led to the colors that Rainbow Petrified Wood is known for.
For example, iron oxide created the signature reds, browns and yellows, but I did also find some really dark almost black pieces and this is likely to have come from carbon.
https://georarities.com/petrified-wood-and-fossils-for-sale/arizona-rainbow-petrified-wood/
https://www.nps.gov/pefo/learn/nature/petrified-wood.htm
https://www.nps.gov/articles/000/permineralization-and-replacement.htm
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