sedimentary

Weathered chert (flint) pebble

Microcrystalline quartz/chalcedony (SiO₂), commonly with iron-oxide staining

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; opaque gray, cream, and reddish-brown coloration with a dull to waxy luster. It has a cryptocrystalline quartz structure, no visible cleavage, conchoidal fracture, and typically high toughness.

Identified August 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica-rich fluids replace or cement sediment, often from marine organisms such as radiolarians or sponges; iron oxides produce the brown-red weathering colors. The specimen’s exact geological age cannot be determined from the photograph.

Uses & applications

Used historically for sharp stone tools and fire-starting, and occasionally as aggregate or decorative stone. Ordinary pebbles have minimal commercial or jewelry value.

Geological facts

Its smooth, rounded shape indicates transport and abrasion in a river, beach, or glacial deposit. A fresh broken surface should be glassy and sharp-edged, unlike softer sandstone or ordinary claystone.

Field identification & locations

Common worldwide, especially in limestone regions and gravel beds; notable sources include the British Isles, the Great Lakes region, Texas, and the Midwest. Test carefully with a steel file or knife: chert generally scratches glass and produces a sharp conchoidal break.