sedimentary
Chert (Field Stone)
Microcrystalline / Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dull gray to brown, Luster: Waxy to dull, Crystal structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dull gray to brown, Luster: Waxy to dull, Crystal structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or by chemical precipitation from groundwater. This specimen appears and displays signs of water-worn erosion, likely from a fluvial or glacial deposit.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) and as a spark generator for flintlock firearms; currently used in construction as aggregate or as decorative gravel.
Geological facts
Chert is so durable that it often survives long after the surrounding rock has weathered away. It was one of the most vital materials for early human technological development.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel blade) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds, glacial tills, and limestone formations.
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