How to identify Chert

To correctly identify Chert (Sedimentary rock (chemical)), 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: white, gray, black, brown, red, green, yellow. Often uniform or banded.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to waxy, sometimes vitreous on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained (aphanitic), smooth to slightly gritty. Non-clastic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline or microcrystalline aggregates of quartz. Individual crystals are 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

   Typically found as nodules, concretions, or beds within carbonate rocks (limestone, dolomite) and shales. Also occurs as bedded deposits in deep marine environments (e.g., radiolarian cherts).

Key Facts

Hardness

7 (Mohs scale)

Specific Gravity

2.58 - 2.64 g/cm³

Crystal System

Trigonal (as quartz, but cryptocrystalline)

Color

Highly variable: white, gray, black, brown, red, green, yellow

Luster

Dull to waxy, sometimes vitreous on fresh fracture

Transparency

Opaque to translucent

Fracture

Conchoidal

Cleavage

None

Composition

Silicon dioxide (SiO2)

Physical characteristics

• Crystal Habit: Cryptocrystalline to microcrystalline aggregates; massive, nodular, or bedded.
• Cleavage Type: Absent
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Dull to waxy, sometimes vitreous

Explore Chert