How to identify Fossiliferous Chert

To correctly identify Fossiliferous Chert (Sedimentary rock (Chert with fossil inclusions)), 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, including white, gray, black, brown, red, green, or variegated, often influenced by impurities and the type of fossils present.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to waxy, vitreous on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Cryptocrystalline to microcrystalline, very fine-grained, smooth to slightly granular. The presence of visible fossil inclusions is the defining textural characteristic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Massive, nodular, or bedded. Individual quartz crystals are microscopic and anhedral.

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 forms in marine environments, particularly deep-sea settings where terrigenous sediment input is low and siliceous organisms thrive. Can also occur in shallow marine or lacustrine settings. Often found as nodules within limestone or dolomite, or as distinct beds within shale or mudstone sequences.

Key Facts

Hardness

6.5-7 on the Mohs scale

Specific Gravity

2.58-2.64 g/cm³

Crystal System

Trigonal (for microcrystalline quartz)

Color

Highly variable, often gray, brown, black, white, red, or green

Luster

Dull to waxy, vitreous on fresh fracture

Transparency

Opaque to translucent

Fracture

Conchoidal

Cleavage

None

Composition

Primarily silicon dioxide (SiO2), with varying amounts of impurities and fossilized organic matter.

Physical characteristics

• Crystal Habit: Cryptocrystalline to microcrystalline aggregates, forming massive, nodular, or bedded structures. Individual quartz crystals are anhedral and microscopic.
• Cleavage Type: Absent
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Dull to waxy, vitreous on fresh fracture surfaces.

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