Petrified vs. Fossilized - What's The Dif?

Posted by Maranda Powers on

Fossilized wood vs petrified wood, what's the difference? Is there a difference?! Turns out it is kind of simple and also kind of complicated.

Petrifaction is a type of fossilization, so all petrified wood is fossilized wood but not all fossilized wood has been petrified. To be considered petrified, the cells of the original organism had to have been replaced by minerals via permineralization so that a record of the internal structures are preserved. This is in contrast to cast fossils where the entire original organism decomposed leaving a cavity which was then filled in with minerals.

That was a lot, so let's get into this a little deeper.

Two examples of fossilized wood, but only the specimen on the right was petrified

As with most fossilization, the best chance a chunk of wood has to be preserved in stone is by being covered very quickly in something that protects it from decaying too fast. River or ocean sediment works great and so does volcanic ash. Both methods surround the wood and protect it from oxygen which would break the material down much too rapidly for our purposes. Just being buried isn’t enough though, the wood needs to be buried underground where water that is jam packed with dissolved minerals can surround it and seep in. This is where the magic, er, the science happens! Permineralization!

The low oxygen environment doesn’t completely stop the wood from decaying, it just lets it happen very very slowly. As those cells decay very very slowly, they leave behind a space which can then be filled with minerals precipitating out of the groundwater. Since different parts of the plant are made of different things, they decay at different rates. The parts of the plant that decay quickest get replaced by minerals first, then the next parts start to decay and are replaced and so on. This is taking place over a super long time so the types of minerals in the groundwater might change or the concentration of the solution may fluctuate. The mineral that replaced the first part to decay may be different from the mineral that replaces the last parts to decay. This fact (and some stuff about chemical bonds that I don’t understand) gives us the color and tone variation in those marvelously preserved wood grain patterns, growth rings and bark texture. This process can preserve the original structure down to the microscopic level! That’s petrification, babyyy! It gives us beautiful things that can also teach us loads of stuff - my favorite combo!

Now let’s look at another scenario. Maybe our ancient log or branch is covered in sediment but decay happens too fast for permineralization, or perhaps the wood is covered in volcanic ash but the heat from the volcanic eruption burns away our organic material. We still have a true to life mold of that original organism encased in hardened sediment or ash with mineral rich water seeping through and depositing material. The difference is that in this situation, the minerals are just filling the void layer by layer starting from the bottom up, not cell by cell since the original material is gone. No internal details are preserved but we still get a super cool fossil out of the deal, it’s just not technically petrified and has less information about the original organic material. We call this type of fossil a cast fossil, and the hardened sediment that surrounds the cast fossil is the mold fossil. 

Above are two fossil palm wood pieces from my shop showing the difference between fossilized and petrified. The earrings on the left are fossils that were not petrified, no interior structures from the original matter were preserved, but minerals did fill the cavity left by the palm tree layer by layer leaving the banded patterns. The pendant on the right is a fossil that was petrified. The original palm tree root material was replaced with minerals cell by cell preserving details of the original plant matter.

 This fossilized vs petrified definition doesn’t just apply to wood. Dino bones, shells, teeth and all sorts of things can be petrified AND can be fossilized without being petrified. I’m not sure why saying petrified wood is more common than saying petrified dinosaur bones - petrified really stuck to wood for some reason. From what I have read, in non-technical-sciencey situations you are safe to use either.


A collection of petrified and fossilized wood pieces from my shop

Wow, this one was a doozy! I hope you enjoyed getting into the nitty gritty on that with me. I really just scratched the surface though, so if this sparked any curiosity I highly recommend doing some further reading, especially reading things by like, people who are scientists and researchers ;) If you can get your hands on Ancient Forests, A Closer Look At Fossil Wood by Frank J Daniels and Richard D Dayvault, your mind will be blown and your eyeballs will be saturated with beautiful images. This book is a masterpiece and is priced accordingly, but someday I am going to treat myself! In the mean time, I was able to check it out from my local library branch in Chicago by doing an inter-library loan, so you may be able to find it in your area too.

'Til next time!



*** I am not a scientist or an expert on any of the topics I cover on this blog, I am just an enthusiastic rock and fossil fan trying to spread the love. I encourage you to do your own reading and research on any topics that might strike your fancy. If you are an expert and would like to gently correct me, please do! You can reach me via email at You can also reach out to tell me how I totally nailed it, or just to say hi, or to show me some cool rock you found.***



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