How to identify Banded Chert

To correctly identify Banded Chert (Sedimentary Chert), 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, often banded with alternating colors such as white, grey, black, brown, red, green, or yellow. The colors are due to trace impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to waxy, sometimes vitreous on freshly fractured surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained (cryptocrystalline), smooth to slightly granular. Non-clastic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline aggregates of quartz; individual crystals are microscopic and not visible to the naked eye.

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

   Deep marine environments (e.g., abyssal plains, continental slopes) where siliceous microfossils accumulate, or in shallow marine settings where silica precipitates from seawater. Also found as nodules and concretions within limestones and dolomites, and as bedded deposits in association with volcanic rocks or Banded Iron Formations.

Key Facts

Hardness

7 (Mohs scale)

Specific Gravity

2.58 - 2.64

Crystal System

Trigonal (for the constituent quartz crystals, though cryptocrystalline)

Color

Highly variable, often banded; white, grey, black, brown, red, green, yellow

Luster

Dull to waxy, sometimes vitreous

Transparency

Opaque to translucent

Fracture

Conchoidal

Cleavage

None

Composition

Primarily Silicon Dioxide (SiO2)

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, forming massive, nodular, or bedded structures.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Dull, waxy, or sub-vitreous

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