sedimentary

Chert (Smooth River Stone)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray with yellowish-brown iron staining; Luster: Waxy to vitreous; Crystal Structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray with yellowish-brown iron staining
Luster: Waxy to vitreous

Identified July 14, 2026More sedimentary →

Explore Chert (Smooth River Stone) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from silica-rich fluids within sedimentary rock layers. This specimen has been smoothed by fluvial (river) erosion over many years.

Uses & applications

Historically used for stone tools/arrowheads due to its sharp edges when broken. Today used in decorative landscaping, aquarium gravel, and as a material for lapidary tumbling.

Geological facts

Chert is extremely durable and chemically stable, which is why it often survives as a smooth pebble in riverbeds long after other rocks have eroded away. Varieties include flint, jasper, and agate.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and conchoidal (shell-like) fracture patterns. It is found globally in sedimentary basins and riverbeds.

Other sedimentary specimens

See all

Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Brown Sandstone (River Rock)

Arenite

sedimentary

Mudstone

Argillite / Siliciclastic sedimentary rock

sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary