How to identify Chert

To correctly identify Chert (Chert (a microcrystalline variety of 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: white, gray, black, brown, red, green, yellow. Often mottled or banded.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy to dull, sometimes vitreous on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Cryptocrystalline to microcrystalline; very fine-grained, smooth to the touch.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Does not exhibit macroscopic crystal forms; occurs as nodules, concretions, or massive beds.

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 evaporites. Also occurs as bedded chert in deep marine environments, often associated with radiolarian or diatomaceous oozes.

Key Facts

Hardness

7 (Mohs scale)

Specific Gravity

2.57 - 2.66 g/cm³

Crystal System

Trigonal (microcrystalline aggregates)

Color

Highly variable (white, gray, black, brown, red, green, yellow)

Luster

Waxy to dull, sometimes vitreous

Transparency

Opaque to translucent

Fracture

Conchoidal

Cleavage

None

Composition

Silicon dioxide (SiO2)

Physical characteristics

• Crystal Habit: Cryptocrystalline to microcrystalline aggregates; typically massive, nodular, or bedded.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Waxy to dull, occasionally vitreous on fresh surfaces.

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