Fossiliferous Chert (with Fossil Shells)

Physical Properties

Hardness: 7 (Mohs scale), Color: Dark grey to brown with light tan fossil cavities, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)

Formation & Geological History

Formed through the replacement of limestone or volcanic ash by silica-rich fluids. The voids and internal structures were created by ancient marine organisms (likely mollusks or brachiopods) whose shells were later partially dissolved or replaced.

Uses & Applications

Primarily used for geological study, specimen collecting, and occasionally as decorative gravel or lapidary material.

Geological Facts

Chert is so hard it was the primary material used by prehistoric humans to make tools like arrowheads and scrapers. This specific piece is a 'trace fossil' host, capturing biological shapes in stone.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and the presence of 'pocked' weathering where softer organic material has eroded away. Commonly found in riverbeds and limestone outcrops.

Identified on: 4/15/2026

Mode: Standard