sedimentary

Chert (often called Flint when in chalk or limestone)

Silicon Dioxide (SiO₂)

Hardness: 7 (Mohs scale); Color: Gray, tan, and blue-gray banding; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Gray, tan, and blue-gray banding
Luster: Dull to waxy

Identified April 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica from water, often replacing organic material or occurring as nodules within carbonate rocks like limestone or chalk. Commonly associated with Paleozoic and Mesozoic sedimentary basins.

Uses & applications

Historically used for stone tools and arrowheads due to its sharp edges; currently used as a raw material for silicon, road aggregate, and for decorative landscaping.

Geological facts

Chert was one of the most important minerals for early humans, used for starting fires (when struck against steel) and crafting sharp weaponry. This specimen shows a distinctive nodular or banded formation.

Field identification & locations

Identify in the field by its very hard surface (can scratch steel/glass) and its smooth, shell-like (conchoidal) breakage pattern. Often found as lumps or nodules protruding from softer limestone outcrops.

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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