sedimentary

Chert (River Cobble)

Microcrystalline or cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale); Color: Brown, tan, and reddish-grey; Luster: Dull to waxy; Structure: Non-crystalline/Massive; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Brown, tan, and reddish-grey
Luster: Dull to waxy

Identified April 30, 2026More sedimentary →

Explore Chert (River Cobble) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 (Mohs scale); Color: Brown, tan, and reddish-grey; Luster: Dull to waxy; Structure: Non-crystalline/Massive; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation. This specific specimen has been chemically weathered and physically rounded into a 'cobble' by water action in a river or creek bed.

Uses & applications

Historically used for stone tools (arrowheads/scrapers) due to its sharp edges when broken. Today used in landscape construction, road base, and as decorative river rock.

Geological facts

Chert is so hard it can scratch steel; when struck against steel, it can create sparks, which led to its historical use in flintlock firearms and fire-starting.

Field identification & locations

Identify by its extreme hardness, smooth water-worn surface, and waxy luster on fresh breaks. Found globally in stream beds, gravel pits, and sedimentary outcrops.