February 4, 2021
Petrified wood is a fossil. It forms when plant material is buried by sediment and protected from decay due to 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.
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 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. One almost unbelievable material from Western Australia is known as “peanut wood” because of its ovoid markings, but those markings are actually boreholes drilled by a clam!
In the United States, noteworthy locations where abundant fossilized wood can be seen include:
Petrified Forest National Park near Holbrook, Arizona
Petrified Palm Deposits in the Catahoula Formation of Louisiana, Texas and Mississippi
Ginkgo Petrified Forest near Wanapum Reservoir, Washington
The Petrified Forest near Calistoga, California
Mississippi Petrified Forest near Flora, Mississippi
The Gilboa Fossils near Gilboa, New York
Florissant Fossil Beds near Florissant, Colorado
Gallatin Petrified Forest near Yellowstone, Wyoming
Escalante Petrified Forest State Park near Escalante, Utah
Petrified Wood Park in Lemmon, South Dakota (a rock sculpture park - some made of local petrified wood)
Blue Forest near Eden Valley, Wyoming
Types of Silicified Wood
Some of the best specimens of petrified wood have been preserved by silicification. Two forms of silicification are common. The most abundant is wood that has been replaced and infilled by chalcedony (sometimes called "agatized wood"). The other form is wood that has been infilled and replaced by opal (usually called "opalized wood"). Both of these varieties can be called "silicified wood" if you are not certain of their identity.
These materials can have a similar appearance that requires testing to positively identify. However, if you have experience in geology or gemology, the tests below are very helpful in separating them. Opalized wood has a lower hardness, a lower refractive index, and a lower specific gravity than chalcedony, as shown in the table below.
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. It can be cut into cabochons or 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.
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 Legally
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 government agency in charge of any government land where collecting will occur.