Sometimes if you are out hiking, if you are lucky, you will stumble across a gemstone which feels like rock, but looks like wood. This is petrified wood.

There are many different types of petrified wood, but before you can identify them, it is important to understand a little more about petrified wood itself.

“Wait,” you may be thinking, “How can wood be a gemstone?”

What is Petrified Wood?

Petrified wood table – Image credit to Deidre Woollard – CC BY 2.0

The explanation is that petrified wood is a fossil. That means that it started out as wood, but over millions of years, that organic material was replaced by minerals.

Usually this doesn’t happen, because microorganisms and oxygen take their toll on dead wood, breaking it down. But when the wood gets buried, it isn’t exposed to oxygen or the action of those microorganisms. That is the wood which gets fossilized.

Petrified wood can be spectacular to behold, because it generally retains the cellular structure of the original wood. So you can see that same beautiful wood grain, but it has been preserved and rendered as stone. All kinds of minerals can be involved in this transformation, so you can see a dazzling array of colors and textures in a single fossil.

While chances are you haven’t found a whole lot of petrified wood just lying around on the ground, it is quite common. Typically, it is found in deposits of sedimentary or volcanic rock. If you happen to be living in the US, you can find petrified wood at locations like the Petrified Forest National Park in Holbrook, AZ, or the Ginkgo Petrified Forest in Washington.

How Many Types of Petrified Wood are There?

You can classify “types” of petrified wood in a couple of ways. You can identify the type of wood which was fossilized in the first place. Or you can identify the minerals which have taken over the structure of the wood.

According to the National Park Service, there are nearly a dozen different types of petrified trees in the Petrified Forest in Arizona, and likely still more species of petrified trees located on the site that researchers have not yet identified. The trees found in the park include ginkgoes, tree ferns, and coniferous species.

Petrified Forest National Park – Image credit to Park Ranger – CC BY 2.0

Many of these species are related to modern species of trees, but remember, these are prehistoric specimens. So if you find a piece of petrified ginkgo, it can be identified as an ancestral relative of modern ginkgo, but it will not be exactly the same. You may also find petrified examples of trees like redwoods which have been around since prehistoric times.

Modern trees can be petrified, and someday, rock hunters will probably discover fossilized examples of all the familiar trees we live among. Interestingly enough, scientists have managed to artificially induce the fossilization process in a laboratory using an acid-leaching method.

This process remarkably took only days, but in nature, the fossilization of wood takes millions of years. In short, any type of wood can be petrified. But what you find in nature while rock hunting will be prehistoric.

Identifying Types of Petrified Wood

Polished petrified wood closeup – click to enlarge – Image credit to Petrified Forest – CC BY 2.0

The best way to figure out what type of petrified wood you are looking at is to examine the cell structure. Unfortunately the cell structure is oftentimes destroyed, but in cases where it is preserved, you have a shot at determining the original species.

To do this, you will probably need a microscope. Using magnification of 10x-800x, you will be able to examine the cells. If you get a chart of cell structures, you can cross-reference and make an educated guess. If for example you see a structure that resembles corn, you are likely looking at gingko. If you see straight rows of round cells, you could be examining a petrified conifer.

You can also look at the rays. These are the spokes of cells which run from the middle of the tree trunk out across the radius to the edge. Rays are yet another hint, since there is quite a bit of variation from one type of tree to the next.

If you see rays that are different widths, you could be looking at a tree which bore fruit. If you see lots of narrow rays that are the same width, it could be a pine tree. Pine trees also may have larger “cells” which are actually resin ducts—another giveaway.

Minerals Commonly Making Up Petrified Wood

Usually, silicate minerals replace the wood in these fossils. Quartz is a common example. Because quartz has a strong cleavage, it is easy for it to split quite evenly. This is why when you are walking through a petrified forest, you will see a lot of fossilized logs which look as if someone has grabbed an axe and simply split them down the middle. This is a puzzling sight to many rock hunters and sightseers. In reality, this is a natural break in the quartz.

Other types of minerals may also be present in petrified wood. You might find petrified wood with calcite or pyrite, for example, or even opal (which is typically a mineraloid, since it contains a large amount of water content and a non-crystalline structure).

What Gives Petrified Wood Its Color?

You may be tempted to try to identify species of trees or minerals on the basis of color, but the huge range of hues found in petrified wood are usually the result of the presence of trace elements. Here is a quick guide:

• Black usually indicates the presence of carbon.
• Green or blue shades usually come from cobalt, copper or chromium.
• Blackish and yellow colors may be the result of manganese oxides.
• Pink or orange result from manganese.
• Red, brown, and yellow shades are contributed by iron oxides.

So when you see those colors, you know what trace elements are present with the minerals. But caution: colors alone do not necessarily reveal the minerals or the original type of wood. That having been said, sometimes those factors are responsible. I have seen pieces of polished, petrified redwood which shared the exact same color as living redwood.

While the colors of petrified wood usually show up beautifully even with rough specimens in their natural form, you can really get them to emerge by polishing the stone. You will find many tumbled and polished specimens for just this reason—plus those work beautifully in jewelry.

But as the natural shape of the wood is part of the appeal for many people, petrified wood is often left in its natural form, but simply polished across the top. This preserves the shape and brings out the colors.

Polished petrified wood sinks – Image credit to Indo Gemstone – CC BY 2.0

What is Jet?

One more type of petrified wood—sort of—is a gemstone called jet. Jet is a beautiful black stone which is lightweight and easy to carve into all sorts of amazing shapes. Jet has been made into jewelry since the Bronze Age, and reached the height of its popularity during Victorian times. When Queen Victoria’s husband died, she started wearing jet while she was in mourning, and the populace imitated her.

Why is jet “sort of” a form of petrified wood? Usually it is described as being a type of fossilized wood, but the process of formation is unique. Unlike other types of petrified wood, jet does not have the original shape and structure of the preserved wood. Jet does start the same way, with dead wood buried underground. But over millions of years, jet undergoes a different process where it is heated and compressed. This actually results in a form of super-compact lignite coal.

Sometimes you will hear jet referred to as “black amber.” This is even more of a misnomer than calling it petrified wood, which at least has some accuracy. Jet has nothing to do with amber.

Now you should have a pretty good starting point if you want to identify a piece of petrified wood. By examining its cellular structure, which has been preserved over a period of millions of years, you can identify what type of prehistoric tree it originated as. You can also identify the minerals and trace elements that replaced the organic matter. You now also know what jet is, and how it is both related to and different from other forms of petrified wood.

Resources:

• http://phys.org/news/2005-01-petrified-wood-days.html
• http://www.evolvingearth.org/learnearthscience/sciencearticles0809identifypetrifiedwood.htm
• http://www.ehow.com/info_8150213_identify-types-petrified-wood.html
• http://www.nps.gov/pefo/faqs.htm
• http://geology.com/stories/13/petrified-wood/
• http://www.petrifiedwood.org.uk/how-long-does-it-take-for-wood-to-become-petrified.html
• https://en.wikipedia.org/wiki/Petrified_wood
• http://www.bwsmigel.info/GEOL.115.ESSAYS/Gemology.Jet.html