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Peanut Wood Jasper is a unique form of petrified wood characterized by distinctive oval or 'peanut-shaped' markings. These markings are fossilized borings made by marine bivalves (often referred to as 'shipworms' or Teredo worms) into driftwood before it became silicified. The wood itself is typically silicified into a dark jasper or chalcedony, ranging from black to dark brown, while the borings are filled with lighter-colored silica, often white, cream, or tan, creating a striking contrast. It is a pseudomorph after wood, meaning the original organic structure has been replaced by mineral matter while retaining the original form.
How to Identify
- Color
- Dominantly dark brown to black matrix with contrasting white, cream, or tan 'peanut-shaped' inclusions. Other colors like reddish-brown or grey may also be present in the matrix.
- Luster
- Dull to waxy or vitreous when polished.
- Texture
- Fine-grained, dense, and smooth when polished. The 'peanut' inclusions may have a slightly different texture or sheen.
- Crystal Form
- Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. It retains the macroscopic structure of the original wood.
- Cleavage
- None, as it is a cryptocrystalline aggregate of quartz.
- Geological Environment
- Marine sedimentary environments where driftwood was infested by marine borers and subsequently buried and silicified. Often found in ancient marine deposits.
Key Facts
- Hardness: 6.5-7 on the Mohs scale
- Specific Gravity: 2.58-2.64
- Crystal System: Trigonal (as cryptocrystalline quartz)
- Color: Dark brown to black matrix with white, cream, or tan 'peanut' inclusions
- Luster: Dull to waxy, vitreous when polished
- Transparency: Opaque
- Fracture: Conchoidal to splintery
- Cleavage: None
- Composition: Silicon dioxide (SiO2) with various impurities
Quick Check
- Color: Dark brown to black with white/cream 'peanut' patterns
- Luster: Dull to waxy
- Streak: White
Physical Characteristics
- Crystal Habit: Massive, cryptocrystalline aggregate, pseudomorph after wood
- Cleavage Type: None
- Fracture Type: Conchoidal to splintery
- Tenacity: Brittle
- Luster Type: Dull to waxy
Formation
Peanut Wood Jasper forms when ancient wood, primarily from the Araucaria tree species, is buried in marine sediments. The wood undergoes permineralization, where organic material is replaced by silica (SiO2) from groundwater. The characteristic 'peanut' patterns are formed by borings of marine bivalves (Teredo worms) into the driftwood before silicification. These borings are subsequently filled with lighter-colored silica or chalcedony, contrasting with the darker, often black or brown, silicified wood matrix.
Usage
Primarily used as an ornamental stone, for lapidary work (cabochons, spheres, carvings), and in jewelry. It is also collected by fossil and mineral enthusiasts due to its unique aesthetic and paleontological significance.
Age Distribution
Cretaceous Period (approximately 120 million years old)
Where to Find
Western Australia
The primary and most well-known source of Peanut Wood Jasper is the Kennedy Range in Western Australia, particularly from the Cretaceous Birdrong Shale formation. It is often found as river-worn pebbles and cobbles.
Finding Tips
Look for distinctive patterns
The most defining characteristic is the 'peanut' or oval-shaped markings against a darker background. These are unmistakable once recognized.
Check for hardness
As a form of jasper (quartz), it will be hard (Mohs 6.5-7) and will scratch glass.
Examine for wood grain
Even though it's silicified, faint remnants of the original wood grain might be visible in the darker matrix, especially on unpolished surfaces.
Consider the geological context
It is found in ancient marine sedimentary environments, often associated with shale or sandstone formations that once contained driftwood.
Similar Rocks
Petrified Wood
Silicified Wood
Also known as: Fossilized Wood
Jasper
Cryptocrystalline Quartz
Also known as: Chert, Agate
Opalized Wood
Opal (SiO2·nH2O) pseudomorph after wood
Also known as: Fossilized Wood (Opal)
Scientific Classification
- Mineral Class
- Silicates (Tectosilicates)
- Group
- Quartz Group
- Crystal System
- Trigonal
- Chemical Formula
- SiO2
- Composition
- Silicon dioxide (SiO2) with trace impurities (e.g., iron oxides, organic matter) that impart color. The 'peanut' inclusions are also SiO2, often chalcedony.
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