sedimentary
Chert Cobble with Crinoid Fossil
Silica (SiO2) with Crinoid ossicle
Mohs hardness: 7; Color: Tan to light brown; Luster: Waxy to dull; Structure: Cryptocrystalline with circular fossil inclusion; Cleavage: None (conchoidal fracture).
- Hardness
- 7
- Color
- Tan to light brown
- Luster
- Waxy to dull
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Physical properties
Mohs hardness: 7; Color: Tan to light brown; Luster: Waxy to dull; Structure: Cryptocrystalline with circular fossil inclusion; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed via chemical precipitation of silica or replacement of limestone in marine environments during the Paleozoic era. The circular inclusion is a fossilized segment of a sea lily (crinoid) stem.
Uses & applications
Used historically for stone tools due to its hardness; currently used in construction aggregate and as a collector's specimen for fossil enthusiasts.
Geological facts
Crinoids are marine animals related to starfish. Though they look like plants, they are echinoderms that have existed for over 450 million years. This specific specimen has been water-worn into a smooth river cobble.
Field identification & locations
Identify by its waxy texture, hardness (cannot be scratched by a steel knife), and the presence of circular or 'Cheerio' shaped fossil cross-sections. Commonly found in riverbeds and gravel pits in limestone-rich regions.
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