sedimentary

Chert (River Pebble)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale; Color: Mottled brown, tan, and grey with iron oxide staining; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Mottled brown, tan, and grey with iron oxide staining
Luster: Waxy to dull

Identified June 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from groundwater. This specimen is water-worn, indicating it was transported by a river or stream over a long period.

Uses & applications

Historically used for stone tools (flintknapping); currently used as architectural aggregate, road surfacing, and as decorative gravel. Some banded varieties are used in lapidary work.

Geological facts

Chert is so hard it can scratch steel and was used by prehistoric humans to create sharp spearheads and knives. It is chemically very stable and resistant to weathering, which is why it often survives as rounded pebbles in riverbeds.

Field identification & locations

Identify in the field by its smooth, waxy feel, conchoidal (shell-like) fracture edges, and extreme hardness; it will not be scratched by a steel knife. Commonly found in riverbeds, glacial tills, and limestone deposits.

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