sedimentary

Breciated Chert

Silicon Dioxide (SiO2) with Clastic Structure

Hardness: 7 (Mohs scale); Color: Cream/Tan matrix with dark grey/black angular fragments; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7.

Hardness: 7 (Mohs scale)
Color: Cream/Tan matrix with dark grey/black angular fragments
Luster: Waxy to dull

Identified June 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the fracturing of pre-existing chert layers (tectonic or collapse forces) followed by cementation by silica-rich fluids. Often dates from the Paleozoic to Mesozoic eras.

Uses & applications

Commonly used in lapidary work for cabochons, landscaping, and as an aggregate in construction. Historically used for making sharp-edged tools.

Geological facts

Breccia derives from the Italian word for 'loose gravel.' Chert is so durable that these rocks often survive transport in riverbeds for millions of years, becoming naturally polished into river stones like the one pictured.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel nail) and the distinct presence of angular 'broken' fragments cemented in a finer matrix. Commonly found in riverbeds, glacial tills, and limestone regions.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary