How to identify Fossiliferous Chert

To correctly identify Fossiliferous Chert (Chert (a variety of microcrystalline quartz) 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, yellow, and mottled. The color is often uniform within a single specimen but can be banded or variegated. Fossils may appear as distinct color variations or textural differences.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to waxy, vitreous (glassy) on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Very fine-grained, smooth to slightly granular. The presence of fossils introduces macroscopic textural variations, such as shell fragments, skeletal structures, or impressions.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Cryptocrystalline; individual quartz crystals are too small to be seen without a microscope. 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 environments, often interbedded with limestone, shale, or sandstone. It can occur as nodules, concretions, or bedded layers. Also found in lacustrine (lake) environments and associated with volcanic ash deposits.

Key Facts

Hardness

6.5-7 on Mohs scale

Specific Gravity

2.58-2.64

Crystal System

Trigonal (microcrystalline aggregates)

Color

Highly variable, often uniform or mottled, with distinct fossil colors/textures

Luster

Dull to waxy, vitreous on fresh fracture

Transparency

Opaque to translucent (thin edges)

Fracture

Conchoidal

Cleavage

None

Composition

SiO2 (Silicon Dioxide) with biogenic inclusions

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates, forming nodules, concretions, or bedded layers. Fossils retain their original biological morphology.
• Cleavage Type: Absent
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Dull, waxy, or vitreous

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