How to identify Fossiliferous Chert

To correctly identify Fossiliferous Chert (Chert with Crinoid Fossils), 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

   Variable, commonly gray, black, brown, tan, or reddish-brown. The crinoid fossils often appear as lighter-colored (white, light gray) inclusions.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to waxy (subvitreous) on fresh fracture surfaces.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Cryptocrystalline (individual crystals too small to be seen without magnification). Smooth to slightly granular feel. The presence of visible crinoid ossicles provides a distinctive texture.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Microcrystalline quartz (chalcedony) forming a dense, interlocking mosaic. Crinoid fossils retain their original skeletal forms, often appearing as circular or star-shaped cross-sections.

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

   Marine sedimentary environments, typically associated with ancient shallow seas where both silica-rich organisms (e.g., sponges) and carbonate-producing organisms (e.g., crinoids) thrived. Often found as nodules or beds within limestone or dolostone formations.

Key Facts

Hardness

6.5-7 on the Mohs scale.

Specific Gravity

2.58-2.64 g/cm³.

Crystal System

Trigonal (for quartz, the constituent mineral).

Color

Highly variable, commonly gray, brown, black, tan, red; often with lighter-colored fossil inclusions.

Luster

Dull to waxy (subvitreous).

Transparency

Opaque to translucent on thin edges.

Fracture

Conchoidal (shell-like, smooth, curved breaks with sharp edges).

Cleavage

None.

Composition

Primarily silicon dioxide (SiO2), with minor impurities and embedded silicified calcium carbonate (calcite) fossils.

Physical characteristics

• Crystal Habit: Cryptocrystalline aggregates of quartz (chalcedony). Fossils retain their original biogenic forms.
• Cleavage Type: Absent.
• Fracture Type: Conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to waxy.

Explore Fossiliferous Chert