sedimentary

Chert (and associated river cobbles)

Silicon dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, white, or brown; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Gray, tan, white, or brown
Luster: Waxy to dull

Identified March 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. These specimens appear to be water-worn cobbles from the Quaternary period.

Uses & applications

Historically used for stone tools (flintknapping); currently used in road construction aggregate, landscaping, and occasionally as a source of silica for glass or industrial abrasives.

Geological facts

Chert is so hard it can scratch steel and was used for centuries as 'flint' to start fires. The sharp edges created by its conchoidal fracture made it the primary material for prehistoric arrowheads and spear points.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture curves, waxy texture when freshly broken, and inability to be scratched by a steel knife. Commonly found in riverbeds, glacial tills, and coastal deposits.

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