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Opalized wood is a type of fossilized wood where the original organic material has been replaced by opal. It is a pseudomorph, meaning it retains the external form and internal structure of the original wood while having a different chemical composition. The opal can be common opal (potch) or, less frequently, precious opal, which exhibits play-of-color. The preservation of the wood's cellular structure, growth rings, and even insect borings can be remarkably detailed, making it a valuable paleontological and geological specimen.
How to Identify
- Color
- Highly variable, including white, gray, brown, black, red, orange, yellow, green, blue, and sometimes iridescent (play-of-color) if precious opal is present. Colors are often banded or mottled, reflecting the original wood structure and mineral impurities.
- Luster
- Vitreous to waxy, resinous, or dull.
- Texture
- Smooth to slightly rough, often exhibiting the texture of the original wood grain. Can be conchoidal where fractured.
- Crystal Form
- Amorphous, lacking a defined crystal structure. It forms as a replacement of organic material, preserving the original wood structure.
- Cleavage
- None, as it is amorphous.
- Geological Environment
- Typically found in sedimentary environments where volcanic ash or silica-rich waters were present, such as ancient floodplains, volcanic ash beds, and lake beds. It is often associated with sandstones, siltstones, and shales.
Key Facts
- Hardness: 5.5-6.5 (Mohs scale)
- Specific Gravity: 1.9-2.3 g/cm³
- Crystal System: Amorphous
- Color: Highly variable, often retaining original wood colors or exhibiting mineral-induced hues; can show play-of-color if precious opal.
- Luster: Vitreous, waxy, resinous, or dull
- Transparency: Translucent to opaque
- Fracture: Conchoidal to uneven
- Cleavage: None
- Composition: Hydrated amorphous silicon dioxide (SiO2·nH2O)
Quick Check
- Color: Variable (white, gray, brown, black, red, orange, yellow, green, blue, iridescent)
- Luster: Vitreous to waxy, resinous, or dull
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, botryoidal, reniform, or as pseudomorphs after wood.
- Cleavage Type: None
- Fracture Type: Conchoidal to uneven
- Tenacity: Brittle
- Luster Type: Vitreous to waxy, resinous, or dull
Formation
Opalized wood forms when plant material, typically wood, is buried in sediment and groundwater rich in dissolved silica. Over long periods, the organic material is replaced cell by cell by silica in the form of opal (amorphous hydrated silicon dioxide). This process, known as permineralization or replacement, preserves the original cellular structure of the wood, often in exquisite detail. The presence of water molecules within the silica structure distinguishes opal from other forms of silica like quartz.
Usage
Primarily used as an ornamental stone, for lapidary work, cabochons, carvings, and decorative objects. High-quality specimens with vibrant colors or strong wood grain patterns are highly prized by collectors. It is also used in educational settings to demonstrate fossilization processes.
Age Distribution
Found in geological formations ranging from the Devonian to the Quaternary periods, with significant occurrences in the Mesozoic and Cenozoic eras.
Where to Find
Petrified Forest National Park, Arizona, USA
Famous for vast deposits of petrified wood, much of which is silicified by quartz, but opalized wood can also be found, particularly in the Chinle Formation.
Queensland, Australia
Known for significant deposits of precious opalized wood, particularly in areas like Lightning Ridge and Coober Pedy, where wood has been replaced by precious opal.
Indonesia
Produces large quantities of petrified wood, including opalized varieties, often with vibrant colors and excellent preservation of wood grain.
Madagascar
A source of beautifully colored petrified wood, some of which is opalized, often exhibiting intricate patterns.
Argentina
Patagonia is home to several petrified forests, including the Sarmiento Petrified Forest, where silicified wood, including opalized forms, is abundant.
Finding Tips
Look for Sedimentary Environments
Focus your search in areas with ancient floodplains, volcanic ash deposits, or lake beds, as these are common settings for the formation of opalized wood.
Identify Wood-like Structures
Look for rock fragments that exhibit the characteristic grain, growth rings, or cellular patterns of wood, even if the material itself appears stony.
Check for Luster and Hardness
Opalized wood will have a vitreous to waxy luster and a hardness of 5.5-6.5 on the Mohs scale, which can help distinguish it from actual wood or other rock types.
Examine for Play-of-Color
If searching for precious opalized wood, carefully observe specimens under varying light conditions for the characteristic iridescent play-of-color.
Consult Geological Maps
Utilize geological maps to identify formations known to contain fossilized wood or silica-rich deposits.
Similar Rocks
Petrified Wood (Quartz variety)
SiO2 (Quartz variety)
Also known as: Silicified Wood (variety of Quartz)
Chert
SiO2 (microcrystalline quartz)
Also known as: Flint, Jasper, Agate
Chalcedony
SiO2 (cryptocrystalline quartz)
Also known as: Agate, Jasper, Carnelian
Scientific Classification
- Mineral Class
- Silicates (specifically, a mineraloid)
- Group
- Opal Group
- Crystal System
- Amorphous
- Chemical Formula
- SiO2·nH2O
- Composition
- Hydrated amorphous silicon dioxide, with varying amounts of water (typically 3-21% by weight) and trace impurities that contribute to color.
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