How to identify Dendritic Opal

To correctly identify Dendritic Opal (SiO2·nH2O), 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 is typically white, gray, cream, pale yellow, or colorless. Dendritic inclusions are black, brown, or reddish-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 to slightly uneven, conchoidal fracture surfaces.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous; does not form true 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 low-temperature hydrothermal veins, as amygdule fillings in volcanic rocks, in sedimentary deposits where silica-rich waters have percolated, and as replacements of organic material or other minerals. Often associated with chalcedony, quartz, and other silica minerals.

Key Facts

Hardness

5.5 - 6.5 (Mohs scale)

Specific Gravity

1.98 - 2.25

Crystal System

Amorphous

Color

Colorless, white, gray, cream, pale yellow, with black, brown, or reddish-brown dendritic inclusions.

Luster

Vitreous to waxy, sometimes resinous.

Transparency

Translucent to opaque.

Fracture

Conchoidal to uneven.

Cleavage

None.

Composition

Hydrated silicon dioxide (SiO2·nH2O) with manganese or iron oxide inclusions.

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, or as vein fillings. Never forms euhedral crystals.
• Cleavage Type: None.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Vitreous, waxy, resinous.

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