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Petrified wood is a fossil in which the organic material of wood has been replaced by minerals, most commonly silica, while retaining the original structure of the wood. It is essentially a stone replica of wood, often displaying the tree's growth rings, bark texture, and even cellular details under magnification. The replacement mineral is typically chalcedony, a microcrystalline variety of quartz, giving it a hardness and durability similar to quartz. Its appearance can vary widely depending on the original wood type, the minerals present during fossilization, and the degree of preservation.
How to Identify
- Color
- Extremely variable, depending on trace mineral impurities. Common colors include shades of brown, gray, black, red, orange, yellow, and sometimes blue or green. Often exhibits multiple colors in bands or patterns.
- Luster
- Vitreous (glassy) to waxy, depending on the specific type of silica (chalcedony or opal) and the polish.
- Texture
- Smooth to slightly granular when broken, but the surface often retains the texture of the original wood, including bark patterns, wood grain, and growth rings. Can feel cool and hard to the touch.
- Crystal Form
- Does not exhibit macroscopic crystal forms; it is a pseudomorph after wood. The silica itself is cryptocrystalline (chalcedony) or amorphous (opal).
- Cleavage
- None, as it is composed of cryptocrystalline quartz or opal.
- Geological Environment
- Typically found in sedimentary environments where rapid burial of plant material occurred, often associated with volcanic ash falls, floodplains, ancient riverbeds, and lake deposits. Common in arid or semi-arid regions where erosion exposes ancient sedimentary layers.
Key Facts
- Hardness: 6.5-7 on the Mohs scale (due to quartz/chalcedony)
- Specific Gravity: 2.58-2.64 (similar to quartz)
- Crystal System: Trigonal (for chalcedony/quartz component), but the overall form is a pseudomorph after wood.
- Color: Highly variable, depending on trace elements (e.g., iron oxides for reds/yellows, manganese for blacks/blues).
- Luster: Vitreous to waxy
- Transparency: Opaque to translucent
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Primarily Silicon Dioxide (SiO2), often with trace impurities of iron, manganese, carbon, copper, chromium, cobalt.
Quick Check
- Color: Variable (brown, gray, red, yellow, black, multi-colored)
- Luster: Vitreous to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Pseudomorph after wood; cryptocrystalline aggregates (chalcedony) or amorphous (opal).
- Cleavage Type: None
- Fracture Type: Conchoidal to splintery
- Tenacity: Brittle
- Luster Type: Vitreous to waxy
Formation
Petrified wood forms when plant material is buried by sediment or volcanic ash, protecting it from decay by oxygen and organisms. Groundwater rich in dissolved minerals (primarily silica from volcanic ash or other silica-rich rocks) then flows through the sediment, permeating the plant cells. Over time, these minerals precipitate out of the solution and replace the organic material cell by cell, a process known as permineralization or replacement. The original cellular structure of the wood is often perfectly preserved. The silica typically forms as chalcedony (cryptocrystalline quartz), but can also be opal or macrocrystalline quartz. The colors are imparted by trace elements present during the silicification process: iron oxides (red, brown, yellow), manganese oxides (black, blue), copper (blue, green), chromium (green, blue), cobalt (blue, green), and carbon (black).
Usage
Petrified wood is 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 elements. It is also a significant fossil, providing valuable information about ancient flora and paleoclimates.
Age Distribution
Ranges from Devonian (approximately 400 million years ago) to recent, 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 with chalcedony and exhibits a wide range of vibrant colors due to iron and manganese oxides.
Gresik, East Java, Indonesia
Known for large logs of petrified wood, often with excellent preservation of cellular structure and rich, earthy tones. These deposits are typically Cenozoic in age.
Patagonia, Argentina
Features extensive petrified forests, particularly in the Sarmiento Petrified Forest National Monument, with large, well-preserved logs from the Jurassic period.
Lescun, France
Contains significant petrified wood occurrences, often with fine detail and varied coloration.
Axel Heiberg Island, Nunavut, Canada
Home to a unique 'fossil forest' from the Eocene epoch, where the wood is not silicified but rather lignified and preserved in an anoxic environment, though some silicified examples exist.
Finding Tips
Look for Sedimentary Environments
Focus on areas with exposed sedimentary rock layers, especially those associated with ancient riverbeds, floodplains, volcanic ash deposits, or lake beds. Arid regions often have better exposure of these formations.
Identify Wood Grain and Rings
Even though it's stone, petrified wood retains the characteristic patterns of wood, including growth rings, bark texture, and sometimes even knot holes or insect borings. Look for these distinctive features.
Check for Hardness
Petrified wood, being mostly quartz, is quite hard (Mohs 6.5-7). It will scratch glass and cannot be easily scratched by a steel knife. This distinguishes it from ordinary wood or softer fossils.
Observe Luster and Fracture
It typically has a vitreous to waxy luster and exhibits a conchoidal to splintery fracture, similar to chert or flint.
Respect Regulations
Collecting petrified wood is often restricted or prohibited in national parks, monuments, and some public lands. Always check local regulations and obtain necessary permits before collecting.
Similar Rocks
Chert
Cryptocrystalline Quartz (SiO2)
Also known as: Flint, Jasper, Agate
Opalized Wood
Hydrated Amorphous Silica (SiO2·nH2O)
Also known as: Opalized Fossil Wood
Jasper
Cryptocrystalline Quartz (SiO2)
Also known as: None
Scientific Classification
- Mineral Class
- Silicate (specifically, a variety of Quartz)
- Group
- Tectosilicates
- Crystal System
- Trigonal (for the constituent quartz/chalcedony)
- Chemical Formula
- SiO2 (with trace impurities)
- Composition
- Silicon dioxide, often with minor amounts of iron oxides, manganese oxides, carbon, and other elements that impart color.
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