How to identify Petrified Wood

To correctly identify Petrified Wood (Silicified 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

   Extremely variable, depending on trace elements present during silicification. Common colors include browns, tans, grays, reds, oranges, yellows, blacks, and occasionally blues or greens. Often exhibits banding 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 weathered.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly rough, often exhibiting the texture of bark or wood grain on external surfaces. Internally, it can be very smooth when polished.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Does not have a distinct crystal form as it is a pseudomorph. The silica minerals (quartz, chalcedony, opal) that replace the wood are typically microcrystalline or amorphous, preserving the original woody structure.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is composed of microcrystalline or amorphous silica. It exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in sedimentary basins, ancient floodplains, volcanic ash deposits, and areas where rapid burial of plant material occurred, followed by groundwater rich in dissolved silica. Common in fossil forests and ancient riverbeds.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to quartz/chalcedony content).

Specific Gravity

2.60-2.65 (similar to quartz).

Crystal System

Trigonal (for quartz/chalcedony components), but the overall structure is a pseudomorph of wood.

Color

Highly variable, depending on trace mineral impurities (e.g., iron oxides, manganese oxides, carbon).

Luster

Vitreous to waxy.

Transparency

Opaque to translucent.

Fracture

Conchoidal to uneven.

Cleavage

None.

Composition

Primarily silicon dioxide (SiO2), often in the form of quartz, chalcedony, or opal. Trace elements (iron, manganese, carbon, copper, chromium, cobalt, etc.) provide color.

Physical characteristics

• Crystal Habit: Pseudomorphic after wood; retains the original macroscopic and microscopic structure of the plant material.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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