sedimentary

Chert (containing Flint and Calcareous inclusions)

Silicon Dioxide (SiO2) with Calcium Carbonate (CaCO3) impurities

Hardness: 7 (Mohs scale); Color: Dark grey to black with tan/white chalky surface; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 (Mohs scale)
Color: Dark grey to black with tan/white chalky surface
Luster: Dull to waxy

Identified March 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica in marine environments, often replacing organic material or filling cavities in limestone/chalk beds. Geologically can range from Precambrian to Holocene.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and fire-starting. Modern uses include road aggregate, decorative landscaping, and occasionally as a source for silicon in semiconductor production.

Geological facts

When struck against steel or another hard rock, chert produces sparks, a property that led to its widespread use in flintlock firearms. It breaks into razor-sharp edges which were the primary cutting tools for prehistoric humans.

Field identification & locations

Identify in the field by its hardness (cannot be scratched by a steel knife) and its curved, shell-like (conchoidal) fractures. Often found in riverbeds, glacial tills, or embedded in limestone outcrops.

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Brecciated Chert (Microcrystalline Silica)

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