How to identify Petrified Wood

To correctly identify Petrified Wood (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, including shades of brown, gray, black, red, orange, yellow, green, blue, and white. Colors are due to trace mineral impurities (e.g., iron oxides for reds/browns, manganese for blacks/blues, copper for greens/blues).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to waxy, depending on the specific silica mineral (quartz, chalcedony, or opal).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly rough, often exhibiting the texture of the original wood grain, including growth rings, knots, and bark patterns. Can be microcrystalline to cryptocrystalline.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically massive, preserving the original woody structure. Microscopic examination reveals fibrous or granular aggregates of quartz, chalcedony, or opal.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is composed of cryptocrystalline silica. It exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

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

Key Facts

Hardness

7 (Mohs scale, due to quartz/chalcedony composition)

Specific Gravity

2.60 - 2.65 (similar to quartz)

Crystal System

Trigonal (for quartz/chalcedony component)

Color

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

Luster

Vitreous to waxy

Transparency

Opaque to translucent

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Primarily silicon dioxide (SiO2), often with trace amounts of iron, manganese, carbon, and other elements that impart color.

Physical characteristics

• Crystal Habit: Massive, pseudomorphic after wood; microcrystalline to cryptocrystalline aggregates.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to waxy

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