How to identify Fossiliferous Chert

To correctly identify Fossiliferous Chert (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, often gray, black, brown, tan, white, or reddish, depending on impurities and the original material. The fossils themselves may be a different color or texture.

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

   Very fine-grained (cryptocrystalline), smooth to slightly granular. The texture will be interrupted by the presence of visible fossils, which may have their own distinct textures.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline aggregates of quartz. Individual crystals are too small to be seen without high magnification. Fossils retain their original biological forms.

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 in marine sedimentary sequences, often interbedded with limestones, shales, or sandstones. It can occur as nodules, concretions, or bedded layers. Common in ancient shallow marine shelf environments, deep-sea environments (where radiolarian oozes accumulate), and areas of volcanic activity providing silica.

Key Facts

Hardness

6.5-7 on the Mohs scale (due to quartz composition)

Specific Gravity

2.58-2.64

Crystal System

Trigonal (for the constituent quartz)

Color

Highly variable, commonly gray, black, brown, tan, white, or reddish, often mottled or banded. The color of the fossils may contrast with the matrix.

Luster

Dull to waxy, vitreous on fresh fracture surfaces.

Transparency

Opaque to translucent (thin edges may be translucent).

Fracture

Conchoidal (shell-like, smooth, curved breaks).

Cleavage

None

Composition

Primarily silicon dioxide (SiO2), with varying amounts of impurities (e.g., iron oxides, clay minerals, organic matter) and fossil remains.

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates of quartz, forming massive, nodular, or bedded structures. Fossils retain their original biological morphology.
• Cleavage Type: Absent
• Fracture Type: Conchoidal, producing sharp edges.
• Tenacity: Brittle
• Luster Type: Dull to waxy, vitreous

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