How to identify Chert

To correctly identify Chert (Sedimentary rock (microcrystalline quartz)), 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, ranging from white, gray, black, brown, red, green, to yellow. Color is often due to impurities.

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

   Smooth to slightly granular, very fine-grained, cryptocrystalline.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Massive, nodular, concretionary, or bedded. Individual crystals are microscopic.

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

   Commonly found as nodules or beds within carbonate rocks (limestone, dolomite), shales, and less frequently in sandstones. Also occurs in deep-sea sediments (radiolarites) and as replacement of other minerals.

Key Facts

Hardness

7 (Mohs scale)

Specific Gravity

2.58 - 2.64 g/cm³

Crystal System

Trigonal (microcrystalline aggregates)

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, massive, nodular, concretionary, bedded
• Cleavage Type: None
• Fracture Type: Conchoidal
• Tenacity: Brittle
• Luster Type: Dull to waxy, sometimes vitreous

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