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Petrified wood is a fossil in which the organic material of wood has been replaced by minerals, most commonly silica, while retaining the original structure of the wood.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, known as permineralization or petrification, results in a stone-like material that often displays the tree's growth rings, bark texture, and even cellular details under magnification. It is not a mineral itself, but rather a pseudomorph of wood after silica minerals.
How to Identify
- Color
- Highly variable, including shades of brown, gray, black, red, orange, yellow, blue, green, and white. Colors are often banded or mottled, reflecting the original wood grain and mineral impurities.
- Luster
- Vitreous (glassy) to waxy, sometimes dull.
- Texture
- Smooth to slightly rough, often exhibiting the texture of the original wood, including bark patterns, growth rings, and wood grain. Can be conchoidal where fractured.
- Crystal Form
- Does not exhibit macroscopic crystal form; it is a pseudomorph after wood. The silica itself is cryptocrystalline (chalcedony/quartz) or amorphous (opal).
- Cleavage
- None, as it is composed of cryptocrystalline or amorphous silica.
- Geological Environment
- Typically found in sedimentary deposits, often associated with ancient floodplains, volcanic ash beds, and areas where rapid burial of plant material occurred, followed by groundwater infiltration rich in dissolved silica.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to quartz/chalcedony)
- Specific Gravity: 2.58-2.64
- Crystal System: Trigonal (for quartz/chalcedony component); pseudomorph, so no inherent crystal system for the 'wood' form.
- Color: Extremely variable, depending on trace mineral impurities (iron oxides, manganese, carbon, copper, chromium).
- Luster: Vitreous to waxy, sometimes dull.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal to uneven.
- Cleavage: None.
- Composition: Primarily silicon dioxide (SiO2), often in the form of chalcedony or microcrystalline quartz. May contain trace amounts of iron, manganese, carbon, copper, chromium, etc.
Quick Check
- Color: Variable (brown, gray, red, yellow, black, etc.)
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Pseudomorph after wood; internal structure is cryptocrystalline or microcrystalline.
- Cleavage Type: None.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to waxy.
Formation
Petrified wood forms when plant material is buried by sediment or volcanic ash and protected from decay by oxygen and organisms. Groundwater rich in dissolved silica (SiO2) then flows through the sediment, replacing the original organic material cell by cell with silica. This permineralization process preserves the original cellular structure of the wood. The silica can precipitate as chalcedony (cryptocrystalline quartz), microcrystalline quartz, or sometimes opal. The color variations are due to trace elements present during silicification: iron oxides (red, brown, yellow), manganese oxides (black, blue), copper (blue, green), chromium (green, blue), and carbon (black).
Usage
Primarily used as an ornamental stone, for lapidary work (cabochons, spheres, carvings), and in jewelry. Larger pieces are used for decorative landscaping, tabletops, and architectural features. It is also of significant scientific interest for paleobotanical studies, providing insights into ancient flora and climates.
Age Distribution
Found in geological formations ranging from the Devonian Period (approximately 400 million years ago) to the Quaternary Period (present day), 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 from ancient conifers (e.g., Araucarioxylon arizonicum) and exhibits a wide range of vibrant colors.
Gresford, New South Wales, Australia
Known for petrified wood from the Permian period, often exhibiting excellent preservation of cellular structures.
Lesbos Petrified Forest, Greece
A UNESCO Global Geopark featuring extensive petrified forest remains from the Miocene epoch, including standing petrified trees.
Patagonia, Argentina
Home to several petrified forests, including the Sarmiento Petrified Forest, with large petrified logs from the Jurassic period.
Axel Heiberg Island, Nunavut, Canada
Contains a significant fossil forest from the Eocene epoch, where wood is preserved in a mummified, rather than fully petrified, state, but also includes silicified examples.
Finding Tips
Look for distinctive wood grain
Even though it's stone, petrified wood often retains the characteristic patterns of wood grain, growth rings, and sometimes bark. This is the primary visual identifier.
Check for hardness
Petrified wood, being mostly quartz, will be hard (Mohs 6.5-7). It will scratch glass and cannot be scratched by a steel knife.
Observe fracture patterns
Due to its chalcedony/quartz composition, petrified wood will exhibit a conchoidal (shell-like) fracture, similar to flint or chert, rather than the fibrous fracture of fresh wood.
Consider the geological context
Search in areas known for sedimentary deposits, especially ancient floodplains, riverbeds, or volcanic ash layers. Erosion often exposes petrified wood in these environments.
Be aware of legal restrictions
Collecting petrified wood is often regulated, especially in national parks, monuments, and some public lands. Always check local regulations before collecting.
Similar Rocks
Chert
Cryptocrystalline Quartz
Also known as: Flint, Jasper, Agate
Opalized Wood
Opal (Hydrated Amorphous Silica) pseudomorph after wood
Also known as: Opalized Fossil Wood
Coal
Carbonaceous rock
Also known as: Fossil Fuel
Scientific Classification
- Mineral Class
- Silicate (specifically, a pseudomorph composed of silica minerals)
- Group
- Quartz Group (specifically, chalcedony/microcrystalline quartz)
- Crystal System
- Trigonal (for the constituent quartz/chalcedony)
- 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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