How to identify Chert

To correctly identify Chert (Sedimentary cryptocrystalline 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 banded or mottled.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Waxy, dull, or vitreous (glassy).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline; individual crystals are too small to be seen without a microscope. Typically 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 deposits in deep marine environments (radiolarites) and lacustrine settings (diatomites).

Key Facts

Hardness

7 (Mohs scale)

Specific Gravity

2.58 - 2.64

Crystal System

Trigonal (cryptocrystalline aggregates)

Color

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

Luster

Waxy, dull, or vitreous

Transparency

Translucent to opaque

Fracture

Conchoidal

Cleavage

None

Composition

Silicon dioxide (SiO2)

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, forming nodules, concretions, or massive beds.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Waxy, dull, or vitreous

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