sedimentary

Chert or flint cobbles

Microcrystalline quartz (SiO₂), commonly with chalcedony

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; tan to brown, dull-to-waxy luster, very fine-grained and noncrystalline to the naked eye. It shows conchoidal fracture, pale edges, and no obvious cleavage; specific gravity is approximately 2.6.

Identified August 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica-rich microfossils or silica-bearing fluids replace or cement sediment, generally during diagenesis in sedimentary basins. The image alone cannot establish its geological age; chert ranges from Precambrian to recent formations.

Uses & applications

Used historically for sharp-edged tools and fire-starting, and today as aggregate, road material, flint, and an occasional lapidary material. Uniform pieces can be polished, though these specimens appear weathered and ordinary.

Geological facts

The curved, shell-like breaks and extremely fine texture are characteristic of chert; “flint” usually refers to dark chert, especially from chalk deposits. Tan coloration commonly reflects iron oxides or other impurities.

Field identification & locations

Common in limestone, chalk, gravel, and glacial deposits worldwide; test with a steel file or glass, which it should scratch, and inspect for conchoidal fracture. The pieces may also be archaeological flakes or naturally broken nodules, but no clear artifact shaping is visible.