sedimentary

Chert (River Pebble)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 Mohs; Color: Brown, tan, and grey with possible iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Hardness: 6
Color: Brown, tan, and grey with possible iron staining
Luster: Waxy to dull

Identified July 15, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Brown, tan, and grey with possible iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Formation & geological history

Formed as a chemical precipitate or through the replacement of limestone by silica-rich fluids. This specific piece is a water-worn river cobble, likely millions of years old, smoothed by hydraulic action.

Uses & applications

Used historically for stone tools due to its sharp edges; today used in construction, road aggregate, and as landscaping gravel. Polished pieces are used in lapidary art.

Geological facts

Chert was one of the most important materials for early humans to create arrowheads, scrapers, and blades. It is essentially a dense, non-banded form of chalcedony.

Field identification & locations

Identify in the field by its smooth, waxy texture and how it chips with shell-like (conchoidal) curves. Found globally in riverbeds and sedimentary basins. It is extremely resistant to weathering.