sedimentary

Rounded flint or chert pebble

Microcrystalline quartz (SiO₂), commonly chalcedony with possible impurities

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; dark gray-brown, waxy to dull luster, very fine-grained cryptocrystalline structure, conchoidal fracture, and no visible cleavage. The smooth form indicates natural water or glacial abrasion, though the surface may be polished.

Identified August 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica-rich fluids replace or fill pore spaces in sedimentary rocks, often from biogenic silica such as radiolarians or sponge spicules. Its exact age cannot be determined from the image; chert occurs from Precambrian to modern deposits.

Uses & applications

Used historically for sharp tools and fire-starting, and today as a lapidary, teaching, and decorative material. Ordinary pebbles have little commercial value.

Geological facts

The translucent brown-gray zones and waxy appearance are typical of flint/chert; white patches may be quartz, calcite, weathering rind, or a minor coating.

Field identification & locations

Identify by testing hardness against glass, observing conchoidal fracture, and checking that it does not fizz strongly in dilute acid. Common in river gravels, glacial deposits, and limestone regions worldwide.