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Petrified wood is a fossil in which the organic material of wood has been replaced by minerals, most commonly silica (SiO2), but sometimes by calcite, pyrite, or other minerals. The replacement process occurs so precisely that the original cellular structure, growth rings, and even bark textures of the wood are preserved. The resulting material is hard and dense, often exhibiting a wide range of colors due to trace elements present during mineralization. It is not a true mineral but a pseudomorph, meaning one mineral has replaced another while retaining the original form.
How to Identify
- Color
- Highly variable, depending on the trace minerals present during petrification. Common colors include browns, grays, reds, oranges, yellows, blacks, and whites. Blue and green are rarer. Often exhibits multiple colors in concentric rings or patterns reflecting the original wood grain.
- Luster
- Vitreous (glassy) to waxy, sometimes dull if unpolished or heavily weathered.
- Texture
- Smooth when polished, but can be rough or granular on unpolished surfaces. Often retains the texture of wood grain, including knots and bark patterns.
- Crystal Form
- Cryptocrystalline (microscopic crystals), typically quartz (chalcedony) or opal. Does not exhibit macroscopic crystal forms.
- Cleavage
- None, as it is cryptocrystalline or amorphous.
- Geological Environment
- Typically found in sedimentary deposits, particularly those associated with ancient floodplains, volcanic ash falls, and lake beds. Common in areas with a history of volcanic activity, which provides a source of silica-rich groundwater. Often found in fluvial (river) and lacustrine (lake) environments where wood was rapidly buried.
Key Facts
- Hardness: 6.5-7 on Mohs scale (for silicified varieties); lower for opalized or calcified varieties.
- Specific Gravity: 2.60-2.65 (for silicified varieties); varies with composition.
- Crystal System: Amorphous or cryptocrystalline (no defined crystal system for the aggregate material).
- Color: Highly variable, often multi-colored with wood grain patterns.
- Luster: Vitreous to waxy.
- Transparency: Opaque to translucent.
- Fracture: Conchoidal to uneven.
- Cleavage: None.
- Composition: Primarily silicon dioxide (SiO2) in the form of chalcedony or opal, with trace impurities (iron oxides, manganese oxides, carbon) causing coloration. Less commonly, calcite (CaCO3) or pyrite (FeS2).
Quick Check
- Color: Variable (browns, reds, grays, yellows, black, white, sometimes blue/green)
- Luster: Vitreous to waxy
- Streak: White (for silicified varieties)
Physical Characteristics
- Crystal Habit: Pseudomorphic after wood; no intrinsic crystal habit.
- 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, protecting it from decay by oxygen and organisms. Groundwater rich in dissolved minerals (most commonly silica, but also calcite, pyrite, or other compounds) flows through the sediment and replaces the original organic material cell by cell. This process, known as permineralization or replacement, preserves the original cellular structure of the wood. The silica often originates from volcanic ash or dissolved silicates in the surrounding rock. The rate of replacement and the specific minerals present influence the final appearance and composition.
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 of significant scientific value for paleontological and paleobotanical studies, providing insights into ancient flora and climates.
Age Distribution
Found in geological formations ranging from the Devonian Period (approximately 419 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 silicified and exhibits a wide array of colors.
Gresford, New South Wales, Australia
Known for significant deposits of petrified wood, often exhibiting excellent preservation of cellular structures.
Patagonia, Argentina
Features extensive petrified forests, particularly in the Sarmiento Petrified Forest, with large trunks from the Tertiary period.
Lescun, Pyrénées-Atlantiques, France
Contains petrified wood deposits, often found in riverbeds and sedimentary layers.
Axel Heiberg Island, Nunavut, Canada
Home to a fossil forest from the Eocene epoch, where wood is preserved in a mummified state (not fully petrified) but also includes some silicified examples.
Yellowstone National Park, Wyoming, USA
Contains multiple petrified forests, some standing upright, formed from volcanic ash falls during the Eocene.
Finding Tips
Look for specific geological formations
Research local geology for sedimentary layers, especially those associated with ancient floodplains, volcanic ash, or lake beds. Areas with a history of volcanic activity are often good prospects.
Examine riverbeds and eroded areas
Water erosion can expose petrified wood. Look along riverbanks, dry washes, and areas where soil has been removed.
Identify wood-like structures
Petrified wood retains the original shape and grain of wood. Look for pieces that resemble logs, branches, or stumps, but are unusually heavy and hard.
Check for characteristic luster and hardness
Petrified wood, especially silicified varieties, will have a glassy to waxy luster and will be hard (scratching steel). It will not burn like regular wood.
Be aware of collection regulations
Many areas, especially national parks and protected lands, prohibit or restrict the collection of petrified wood. Always check local regulations before collecting.
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
- Not a true mineral, but a pseudomorph composed primarily of silicates (quartz group or mineraloid opal).
- Group
- Pseudomorph (after wood)
- Crystal System
- Amorphous or cryptocrystalline (for the replacing minerals).
- Chemical Formula
- SiO2 (for silicified varieties); variable depending on replacing minerals.
- Composition
- Silicon dioxide (SiO2) is the dominant component, often with minor amounts of iron, manganese, carbon, and other elements that impart color. The original organic material is entirely replaced.
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