How to identify Chert

To correctly identify Chert (Chert), 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. Color is often uniform within a single specimen but can be banded or mottled.

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 (cryptocrystalline), smooth to slightly granular. Feels smooth and hard.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline; individual quartz crystals are microscopic and not visible to the naked eye. 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

   Commonly found in marine sedimentary environments, particularly associated with limestones, dolomites, and shales. Also occurs in some volcanic settings where silica-rich fluids are present.

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

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 aggregates, forming nodules, concretions, or massive beds.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Dull to waxy, sometimes vitreous

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