sedimentary

Chert or flint fragment

Microcrystalline quartz (SiO₂), commonly chalcedony with possible clay/iron-oxide impurities

AI-generated identification · may be incorrect

Typically Mohs hardness 6.5–7, opaque gray-brown to reddish-brown, with a dull to waxy luster and no visible cleavage; conchoidal fracture and sharp edges are characteristic. The image appears to show a weathered, possibly worked fragment with a smoother interior and rough cortex.

Identified August 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Chert forms when silica-rich groundwater replaces or precipitates within carbonate sediments, often during diagenesis; flint is a dark variety commonly associated with chalk. It may range from Paleozoic to Cenozoic in age, depending on its source bed.

Uses & applications

Used historically for cutting tools, arrowheads, fire-starting, and occasionally decorative stone; modern uses are limited to aggregate, landscaping, and collecting. Archaeologically worked pieces can have additional value.

Geological facts

Freshly fractured chert can produce extremely sharp edges and sparks against steel. Brown or red coloration commonly reflects iron oxides, while the rough outer rind is weathered cortex.

Field identification & locations

Look for a glassy-to-waxy texture, conchoidal fracture, very hard surfaces, and inability to scratch with a steel knife; quartzite and jasper are common look-alikes. Chert occurs widely in limestone, chalk, and sedimentary gravel deposits.