How to identify Dendritic Agate

To correctly identify Dendritic Agate (SiO2 (chalcedony with manganese or iron 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 is typically colorless, white, gray, or pale blue chalcedony. The dendritic inclusions are black, dark brown (manganese oxides), or reddish-brown to orange (iron oxides).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to waxy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to slightly granular on unpolished surfaces; conchoidal fracture surfaces are smooth.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline, meaning individual crystals are too small to be seen without high magnification. Forms botryoidal, mammillary, or stalactitic masses, or fills veins and cavities.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None. Exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Typically found in volcanic rocks (e.g., basalts, rhyolites) where silica-rich fluids have filled vesicles and fractures. Also occurs in sedimentary rocks, particularly limestones and sandstones, and in hydrothermal veins.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to chalcedony).

Specific Gravity

2.58-2.64.

Crystal System

Trigonal (for the constituent quartz microcrystals).

Color

Colorless, white, gray, or pale blue chalcedony with black, brown, or reddish-brown dendritic inclusions.

Luster

Vitreous to waxy.

Transparency

Translucent to opaque.

Fracture

Conchoidal.

Cleavage

None.

Composition

Silicon dioxide (SiO2) with inclusions of manganese oxides (e.g., pyrolusite, psilomelane) or iron oxides (e.g., goethite).

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, typically massive, botryoidal, mammillary, or stalactitic. Individual quartz crystals are microscopic.
• Cleavage Type: None.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to waxy.

Explore Dendritic Agate