sedimentary

Banded chert pebble

Chert (microcrystalline quartz, SiO₂), likely ribbon-banded chert

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; dark blue-gray to black with cream silica-rich bands, smooth waxy to dull luster, very fine-grained cryptocrystalline structure, and no visible cleavage. It appears naturally water-worn or tumbled.

Identified August 30, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when microcrystalline quartz replaces or precipitates from silica-rich sediment, commonly in deep-marine beds; the alternating bands reflect changing sediment or chemical conditions. Its exact geologic age cannot be determined from the photograph.

Uses & applications

Used historically for sharp tools and fire-starting because it fractures conchoidally; attractive pieces are sold as lapidary stones, worry stones, or decorative pebbles. Common material generally has modest commercial value.

Geological facts

The pale stripes may be original sedimentary laminations or veins of lighter chert/quartz. Chert can resemble jasper, basalt, or slate, but its hardness and glassy conchoidal fracture are diagnostic.

Field identification & locations

Common in sedimentary terrains, river gravels, and glacial deposits worldwide. Test cautiously with a steel file or glass: genuine chert typically scratches glass and produces a sharp, curved fracture; exact identification requires testing.