How to identify Dendritic Opal

To correctly identify Dendritic Opal (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

   Base color typically white, gray, cream, or light brown, with black or dark brown dendritic inclusions.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy to resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly uneven, depending on the polish and presence of surface irregularities.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous (mineraloid), lacking a defined crystal structure. Occurs as masses, nodules, or vein fillings.

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 and volcanic rocks, often in cavities, fissures, or as replacements of organic material. It forms in environments where silica-rich waters are present, allowing for the deposition of opal, and where manganese or iron-rich solutions can subsequently infiltrate and precipitate.

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

1.98 - 2.25 (lower than quartz due to water content).

Crystal System

Amorphous (mineraloid).

Color

Translucent to opaque white, gray, cream, or light brown base with black or dark brown dendritic inclusions.

Luster

Vitreous to waxy to resinous.

Transparency

Translucent to opaque.

Fracture

Conchoidal to uneven.

Cleavage

None.

Composition

Hydrated amorphous silica (SiO2·nH2O) with inclusions of manganese oxides (e.g., MnO2) or iron oxides.

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, or as vein fillings. The dendritic patterns are internal inclusions, not external crystal forms.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy to resinous.

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