sedimentary

Chert (often called Jasper or River Siltstone)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 Mohs; Color: Brown, reddish-brown with white quartz veining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 6
Color: Brown, reddish-brown with white quartz veining
Luster: Waxy to dull

Identified July 5, 2026More sedimentary →

Explore Chert (often called Jasper or River Siltstone) in the encyclopedia →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich microfossils or chemical precipitation within sedimentary layers. This specific water-worn pebble likely weathered out of a host rock and was smoothed by fluvial action in a stream or river bed.

Uses & applications

Used historically for stone tools due to sharp edges when fractured; modern use is primarily for landscaping, aquarium decoration, and as a collector's river pebble.

Geological facts

Chert is so durable that it can survive millions of years of transport in rivers. When struck against steel, it can create sparks, which led to its use in flintlock firearms.

Field identification & locations

Identify in the field by its waxy luster, smoothness, and the presence of thin quartz veins. Common in riverbeds and gravel pits globally. It will scratch glass but cannot be scratched by a steel knife.

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)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Sandstone

Arenite (primarily SiO2)

sedimentary