How to identify Petrified Wood

To correctly identify Petrified Wood (Silicified wood (Chalcedony/Quartz)), 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, including shades of brown, gray, black, red, orange, yellow, blue, green, and white. Colors are often banded or mottled, reflecting the original wood grain and mineral impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to waxy, sometimes dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly rough, often exhibiting the texture of the original wood, including bark patterns, growth rings, and wood grain. Can be conchoidal where fractured.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Does not exhibit macroscopic crystal form; it is a pseudomorph after wood. The silica itself is cryptocrystalline (chalcedony/quartz) or amorphous (opal).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is composed of cryptocrystalline or amorphous silica.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in sedimentary deposits, often associated with ancient floodplains, volcanic ash beds, and areas where rapid burial of plant material occurred, followed by groundwater infiltration rich in dissolved silica.

Key Facts

Hardness

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

Specific Gravity

2.58-2.64

Crystal System

Trigonal (for quartz/chalcedony component); pseudomorph, so no inherent crystal system for the 'wood' form.

Color

Extremely variable, depending on trace mineral impurities (iron oxides, manganese, carbon, copper, chromium).

Luster

Vitreous to waxy, sometimes dull.

Transparency

Opaque to translucent.

Fracture

Conchoidal to uneven.

Cleavage

None.

Composition

Primarily silicon dioxide (SiO2), often in the form of chalcedony or microcrystalline quartz. May contain trace amounts of iron, manganese, carbon, copper, chromium, etc.

Physical characteristics

• Crystal Habit: Pseudomorph after wood; internal structure is cryptocrystalline or microcrystalline.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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