sedimentary

Banded chert (possibly flint)

Microcrystalline quartz (SiO₂), chiefly chalcedony and quartz

AI-generated identification · may be incorrect

Typically Mohs hardness 6.5–7, opaque to slightly translucent, black to gray with brown iron-oxide staining, waxy to dull luster, and no visible cleavage. The rounded, polished surface and thin pale bands are consistent with silica-rich chert; exact identification would benefit from a fresh fracture or hardness test.

Identified September 9, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when microcrystalline silica replaces sediment or precipitates from silica-rich marine fluids, often during the Paleozoic–Mesozoic. This specimen was likely naturally water-rounded or tumbled after formation.

Uses & applications

Used historically for sharp tools and fire-starting; today it is mainly a teaching, lapidary, or decorative specimen. It has limited jewelry value unless strongly patterned and well polished.

Geological facts

Conchoidal fracture, sharp edges when freshly broken, and resistance to scratching by steel are characteristic. Dark coloration commonly reflects organic carbon, sulfides, or iron/manganese impurities.

Field identification & locations

Common in limestone beds, river gravels, and glacial deposits worldwide, especially Europe, North America, and Australia. Test with a steel file or glass, while noting that surface coatings can affect results.