How to identify Fossiliferous Chert

To correctly identify Fossiliferous Chert (Chert with fossil impression), 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 uniform within a single specimen, but can be mottled or banded.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Cryptocrystalline (grains too fine to be seen with the naked eye), very fine-grained, smooth to slightly granular. The key identifying feature is the presence of visible fossil structures or impressions.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Massive, nodular, 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 biogenic silica accumulates, or in shallow marine settings where silicification of carbonate sediments occurs. Can also be found in lacustrine (lake) environments. Often associated with limestone, shale, and evaporite deposits.

Key Facts

Hardness

6.5-7 on the Mohs scale

Specific Gravity

2.58-2.64

Crystal System

Trigonal (for quartz, the primary mineral)

Color

Highly variable, often uniform or banded

Luster

Dull to waxy to vitreous

Transparency

Opaque to translucent (thin edges)

Fracture

Conchoidal

Cleavage

None

Composition

Primarily silicon dioxide (SiO2), with minor impurities and fossil remains.

Physical characteristics

• Crystal Habit: Cryptocrystalline, massive, nodular, or bedded aggregates of microcrystalline quartz.
• Cleavage Type: None
• Fracture Type: Conchoidal, producing very sharp edges.
• Tenacity: Brittle
• Luster Type: Dull, waxy, or vitreous

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