How to identify Opalized Wood

To correctly identify Opalized Wood (Silicified wood (variety of Opal)), 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 white, gray, brown, black, red, orange, yellow, green, blue, and sometimes iridescent (play-of-color) if precious opal is present. Colors are often banded or mottled, reflecting the original wood structure and mineral impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, resinous, or dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly rough, often exhibiting the texture of the original wood grain. Can be conchoidal where fractured.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous, lacking a defined crystal structure. It forms as a replacement of organic material, preserving the original wood structure.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is amorphous.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in sedimentary environments where volcanic ash or silica-rich waters were present, such as ancient floodplains, volcanic ash beds, and lake beds. It is often associated with sandstones, siltstones, and shales.

Key Facts

Hardness

5.5-6.5 (Mohs scale)

Specific Gravity

1.9-2.3 g/cm³

Crystal System

Amorphous

Color

Highly variable, often retaining original wood colors or exhibiting mineral-induced hues; can show play-of-color if precious opal.

Luster

Vitreous, waxy, resinous, or dull

Transparency

Translucent to opaque

Fracture

Conchoidal to uneven

Cleavage

None

Composition

Hydrated amorphous silicon dioxide (SiO2·nH2O)

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, or as pseudomorphs after wood.
• Cleavage Type: None
• Fracture Type: Conchoidal to uneven
• Tenacity: Brittle
• Luster Type: Vitreous to waxy, resinous, or dull

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