sedimentary
Fossiliferous Chert (Chert with Crinoid Stem Impression)
Chert (SiO2) with fossil Anthozoan or Crinoid columnals
Hardness: 7 (Mohs scale); Color: Tan, brown, and grey; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: none (conchoidal fracture).
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, and grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, and grey; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: none (conchoidal fracture).
Formation & geological history
Formed through the replacement of limestone or carbonate sediment by silica (silicification). This specimen likely dates to the Paleozoic era (approx. 250-500 million years ago) and formed in a shallow marine environment.
Uses & applications
Used primarily as decorative specimens for collectors; historically used for toolmaking (stone arrowheads) due to its conchoidal fracture.
Geological facts
The ladder-like indentation is a negative mold of a crinoid stem, often nicknamed 'sea lilies'. While they look like plants, they are actually marine animals related to starfish.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and the presence of fossil molds. Commonly found in limestone outcrops, riverbeds, and gravel pits in the Midwest USA.
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