Fossiliferous Chert with Brachiopod Impression

Physical Properties

Hardness: 7 (Mohs scale), Color: tan, grey, and cream, Luster: waxy to dull, Structure: microcrystalline quartz, Cleavage: none (conchoidal fracture), Specific Gravity: 2.65

Formation & Geological History

Formed through the replacement of limestone or carbonate minerals by silica-rich fluids in marine environments. The fossil imprint suggests a Paleozoic age (approx. 250-500 million years old) when brachiopods were dominant.

Uses & Applications

Primarily of value to amateur paleontologists and rock collectors. Historically used for stone tools (lithics). Modern use is limited to aggregate or decorative landscaping rock.

Geological Facts

This specimen contains a partial 'mold' or imprint of a brachiopod shell, which looks like a fan or butterfly wings. Brachiopods are not clams; they are a distinct phylum that has survived through five mass extinctions.

Field Identification & Locations

Look for the sharp, conchoidal (glass-like) edges where the rock has chipped, and the fan-shaped ribbing of the fossil. Commonly found in riverbeds or gravel pits in limestone-rich regions like the Midwest USA.

Identified on: 5/3/2026

Mode: Standard