How to identify Chert

To correctly identify Chert (Chert (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; commonly gray, white, black, brown, red, green, or tan. Color is often due to trace impurities (e.g., iron oxides for red/brown, organic matter for black).

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. Feels smooth and hard.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Does not exhibit macroscopic crystal forms; composed of microscopic quartz crystals. Occurs as nodules, concretions, or bedded layers.

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 or beds within carbonate rocks (limestone, dolomite) and shales. Also occurs as bedded deposits in deep marine environments, often associated with radiolarian or diatomaceous oozes. Can be found in residual soils where carbonate host rocks have weathered away.

Key Facts

Hardness

7 (Mohs scale)

Specific Gravity

2.58 - 2.64

Crystal System

Trigonal (for individual quartz crystals, but cryptocrystalline aggregate)

Color

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

Luster

Dull to waxy, sometimes vitreous

Transparency

Opaque to translucent

Fracture

Conchoidal

Cleavage

None

Composition

Silicon dioxide (SiO2)

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregate; no macroscopic crystal habit. Occurs as nodules, concretions, or massive beds.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Dull, waxy, or subvitreous

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