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Petrified wood is a fossil in which the organic material of wood has been replaced by minerals, most commonly silica, while retaining the original structure of the wood. This process results in a stone-like material that often displays the growth rings, bark texture, and even cellular details of the original tree. The colors vary widely depending on the trace minerals present during petrification, such as iron oxides (red, brown, yellow), manganese oxides (black, blue), and copper (green, blue).
How to Identify
- Color
- Extremely variable, ranging from white, gray, black, brown, red, orange, yellow, green, blue, to purple. Colors are often banded or mottled, reflecting the original wood grain and the presence of various trace elements.
- Luster
- Vitreous (glassy) to waxy, depending on the degree of silicification and the specific mineral replacing the wood.
- Texture
- Smooth to slightly rough, often exhibiting the texture of bark or wood grain. Can be finely crystalline to cryptocrystalline.
- Crystal Form
- Pseudomorph after wood; retains the external and internal structure of the original tree, including growth rings, knots, and cellular details. Does not exhibit its own crystal habit.
- Cleavage
- None, as it is typically composed of cryptocrystalline quartz or chalcedony.
- Geological Environment
- Typically found in sedimentary environments where rapid burial of plant material occurred, such as floodplains, volcanic ash deposits, and ancient riverbeds. Common in areas with past volcanic activity that provided silica-rich groundwater.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to quartz composition)
- Specific Gravity: 2.60-2.65 (similar to quartz)
- Crystal System: Trigonal (for the quartz component), but the overall form is a pseudomorph after wood.
- Color: Highly variable, depending on trace elements (e.g., iron oxides for reds/browns, manganese for blacks/blues).
- Luster: Vitreous to waxy.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal to uneven.
- Cleavage: None.
- Composition: Primarily silicon dioxide (SiO2), often in the form of chalcedony or opal. Trace elements contribute to color.
Quick Check
- Color: Variable, often earthy tones with visible wood grain
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Pseudomorph after wood; retains the original macroscopic and microscopic structure of the plant material.
- Cleavage Type: Absent.
- Fracture Type: Conchoidal to uneven, typical of quartz.
- Tenacity: Brittle.
- Luster Type: Vitreous (glassy) to waxy.
Formation
Petrified wood forms when plant material is buried rapidly under sediment, protecting it from decay by oxygen and organisms. Groundwater rich in dissolved minerals, primarily silica (SiO2) from volcanic ash or other sources, then permeates the porous wood tissue. Over long periods (millions of years), the silica precipitates out of the solution and replaces the organic cell walls, molecule by molecule, a process known as permineralization or replacement. The original cellular structure of the wood is often perfectly preserved, creating a pseudomorph where the original organic material is replaced by mineral matter. Other minerals like calcite, pyrite, or opal can also replace wood, but silica is the most common.
Usage
Petrified wood is primarily used as an ornamental stone for decorative objects, sculptures, tabletops, and jewelry. Larger pieces are sought after for landscaping and as museum exhibits. It is also collected by hobbyists and paleontologists for its scientific value in understanding ancient flora and climates.
Age Distribution
Found in geological formations ranging from the Devonian Period (approximately 400 million years ago) to the Quaternary Period, with significant occurrences in the Triassic, Jurassic, and Tertiary periods.
Where to Find
Petrified Forest National Park, Arizona, USA
World-renowned for its vast deposits of petrified wood, primarily from the Triassic Chinle Formation. The wood is predominantly silicified and exhibits a wide range of vibrant colors.
Gresik, East Java, Indonesia
Known for large, well-preserved petrified wood logs, often with intricate details and rich coloration, dating back to the Miocene epoch.
Patagonia, Argentina
Features extensive petrified forests, particularly in the Sarmiento Petrified Forest, with large logs from the Jurassic period.
Lescun, France
Contains petrified wood from the Permian period, offering insights into ancient European flora.
Axel Heiberg Island, Nunavut, Canada
Home to a fossil forest from the Eocene epoch, where petrified wood is found in situ, providing valuable paleobotanical data.
Finding Tips
Research Local Geology
Identify areas known for sedimentary deposits, especially those associated with ancient floodplains, volcanic ash, or river systems. Geological maps and local geological surveys are excellent resources.
Look for Specific Environments
Petrified wood is often found in eroded badlands, riverbeds, and areas where sedimentary layers are exposed. Look for areas with evidence of past plant life and rapid burial.
Observe Surface Features
Keep an eye out for rock fragments that exhibit wood grain, growth rings, or bark-like textures. The weight of petrified wood is often greater than that of ordinary wood due to mineral replacement.
Check for Hardness
Petrified wood, being largely quartz, will be significantly harder than unpetrified wood. It will scratch glass and cannot be scratched by a steel knife.
Legal and Ethical Considerations
Always be aware of local regulations regarding fossil collection. Many areas, especially national parks and protected lands, prohibit or restrict the removal of petrified wood. Obtain necessary permits if required and practice ethical collecting by leaving enough for others and for scientific study.
Similar Rocks
Chert
Cryptocrystalline Quartz
Also known as: Flint, Jasper, Agate
Opal
Hydrated Amorphous Silica
Also known as: Precious Opal, Common Opal
Chalcedony
Cryptocrystalline Quartz
Also known as: Agate, Jasper, Carnelian
Scientific Classification
- Mineral Class
- Silicate (specifically, a pseudomorph composed of quartz/chalcedony)
- Group
- Quartz group (cryptocrystalline varieties)
- Crystal System
- Trigonal (for the constituent quartz)
- Chemical Formula
- SiO2 (with trace impurities)
- Composition
- Silicon dioxide, often with minor amounts of iron, manganese, carbon, and other elements that impart color.
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