Chert (River Cobble)

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.

Identified on: 4/30/2026

Mode: Standard