What is Petrified Wood?
Petrified wood is a fossil. It forms when plant material is buried by sediment and protected from decay by oxygen and
organisms. Then, groundwater rich in dissolved solids flows through the sediment replacing the original plant material
with silica, calcite, pyrite or another inorganic material such as opal. The result is a fossil of the original woody material that
often exhibits preserved details of the bark, wood and cellular structures.
Some specimens of petrified wood are such accurate preservations that people do not realize they are fossils until they pick
them up and are shocked by their weight. These specimens with near perfect preservation are unusual; however, specimens that
exhibit clearly recognizable bark and woody structures are very common.
 | Some petrified logs contain a spectacular surprise. Cavities within them served as crystallization locations for quartz crystals such as the citrine (yellow, left) and amethyst (purple, right) shown here. Images by Petrified Forest National Park. |
Petrified Forest National Park
The most famous locality for observing petrified wood is Petrified Forest National Park near the community of Holbrook in
northeastern Arizona. About 225 million years ago this area was a lowland with a tropical climate and covered by a dense
forest. Rivers flooded by tropical rain storms washed mud and other sediments into the lowlands. Enormous coniferous trees
up to 9 feet in diameter and 200 feet tall lived and died in these lowlands. Fallen trees and broken branches were often buried by
the river sediments. Nearby volcanoes erupted numerous times. These eruptions blanketed the area in volcanic ash with a high
silica content.
Rapid burial allowed the plant debris to escape destruction by oxygen and insects. The soluble ash was dissolved by groundwater
flowing through the sediments. The dissolved ash served as a source of silica that replaced the plant debris, creating petrified
wood. Trace amounts of iron, manganese and other minerals were included in the silica and gave the petrified wood a variety of
colors. These sediments, plant debris and volcanic ash became part of a rock unit known today as the Chinle Formation.
In the millions of years after the Chinle Formation was deposited the area was uplifted and the rocks deposited above the Chinle
were been eroded away. The petrified wood is much harder and resistant to weathering than the mud rocks and ash deposits of the
Chinle. Instead of eroding away the wood accumulated on the ground surface as the surrounding mud rocks and ash layers were eroded away.
That is why areas of the Park are covered with a litter of petrified wood trunks, branches and fragments. Today, visitors to the
park can observe the petrified wood and photograph it; however, collecting petrified wood in the park is prohibited.
Other Petrified Wood Localities
Petrified wood is not rare. It is found in volcanic deposits and sedimentary rocks at many of locations worldwide. It is sometimes
found where volcanic activity covered plant material with ash, mudflows or pyroclastic debris. It is found where wood in sedimentary
deposits was replaced by minerals precipitated from groundwater. It is especially abundant around coal seams, although many of the
wood specimens in these locations are casts and molds rather than petrifications.
In the United States, noteworthy locations where abundant fossilized wood can be seen include:
Petrified Wood by Many Other Names
A wide variety of names are commonly used for petrified wood. "Fossilized wood" is a general term for wood that has been petrified
or preserved by other methods of fossilization. "Opalized wood" is petrified wood that has been replaced by opal, an amorphous form
of silica. "Agatized wood" is wood that has been replaced by agate, a form of chalcedony or microcrystalline quartz.
"Silicified wood" is wood that has been replace by any form of silica, including opal and agate.
Lapidary Uses of Petrified Wood
Petrified wood is often used in lapidary work. It is cut into shapes for making jewelry, sawn into blocks to make bookends, sawn into
thick slabs to make table tops, and sawn into thin slabs for clock faces, cut into cabochons, used to make tumbled stones and many other crafts. Small pieces of petrified
wood can be placed in a rock tumbler to make tumbled stones.
| A nice piece of petrified wood suitable for lapidary work. The pore spaces in the wood have been completely silicified and the piece is relatively free of fractures. It also has nice color. Petrified wood like this is very hard to find. Specimen is about three inches across. |
Only a small fraction of petrified wood is suitable for lapidary work. Poorly preserved specimens, those with lots of voids or closely-spaced
fractures do not polish well or break while being worked. Specimens with no fractures or voids and with spectacular color are highly prized for lapidary work.
Collecting petrified wood can only be done on private property where permission has been obtained from the landowner or on limited tracts of government lands where small quantities are allowed to be collected for personal use. Before you collect get permission and collecting rules from the owner of private property or from the agency in charge of any government land where collecting will occur.
Contributor: Hobart King
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 | Photograph of a polished cross-section of a petrified log from Arizona. Enlarge the image to see the structure of the wood and even insect borings. Image by Michael Gäbler used here under a Creative Commons license. Enlarge image. |
 | An accumulation of petrified logs in a gulley at Petrified Forest National Park. At the top of the gulley a "pedistal log" is suspended on a column of Chinle Formation. When the Chinle weathers away the log will be lowered to the ground surface. Image by Petrified Forest National Park. |
| A nice piece of petrified wood representing a limb cross section. This specimen shows the growth rings, cellular structure and external bark. Specimen is about six inches across. |
 | Petrified wood with spectacular colors and full petrification is highly prized for lapidary work. It can be polished and used to make jewelry and many other crafts. Image by Petrified Forest National Park. |
 | One of the most popular lapidary activities that uses petrified wood is rock tumbling. Small pieces of petrified wood that are free of pores and fractures are placed in a rock tumbler and tumbled with successively finer abrasives and finally with a rock polish. The results are polished pieces of petrified wood in baroque shapes showing the color and grain of the wood. The pieces of petrified wood in the photo above range in size from about 1/4 inch to 1 inch. |
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