How to identify Petrified Wood

To correctly identify Petrified Wood (Petrified Wood), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to waxy, sometimes dull if unpolished or heavily weathered.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth when polished, but can be rough or granular on unpolished surfaces. Often retains the texture of wood grain, including knots and bark patterns.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline (microscopic crystals), typically quartz (chalcedony) or opal. Does not exhibit macroscopic crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is cryptocrystalline or amorphous.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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.

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