How to identify Dendritic Opal

To correctly identify Dendritic Opal (SiO2·nH2O with manganese dendrites), 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

   Opal matrix is typically white, gray, cream, or light brown. Dendrites are dark, usually black or dark brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy, sometimes resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth, conchoidal fracture surfaces. The dendrites are internal patterns, not surface texture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; does not form crystals. Occurs as botryoidal, reniform, stalactitic, or massive aggregates filling cavities or veins.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its amorphous nature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in sedimentary rocks, volcanic rocks (especially rhyolites and basalts), and hot spring deposits where silica-rich waters precipitate. It often occurs in nodules, veins, or as replacements of organic material.

Key Facts

Hardness

5.5 - 6.5 on the Mohs scale (due to its amorphous nature and water content, it's softer than quartz).

Specific Gravity

1.9 - 2.3 (lower than quartz due to water content).

Crystal System

Amorphous (lacks a crystal structure).

Color

Opal matrix: white, gray, cream, light brown. Dendrites: black, dark brown.

Luster

Vitreous to waxy, sometimes resinous.

Transparency

Translucent to opaque.

Fracture

Conchoidal (shell-like).

Cleavage

None.

Composition

Hydrous silicon dioxide (SiO2·nH2O) with inclusions of manganese oxides (e.g., MnO2).

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, or as vein fillings. No true crystal habit due to amorphous nature.
• Cleavage Type: None.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

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