sedimentary

Black flint (chert)

Microcrystalline quartz/chalcedony (SiO₂)

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; dark gray to black, waxy or dull luster, very fine-grained and cryptocrystalline, with conchoidal fracture and no cleavage. The surface appears naturally weathered and iron-stained, possibly with thin quartz veins.

Identified August 20, 2026More sedimentary →

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Physical properties

Formation & geological history

Flint is a dense variety of chert formed mainly from silica-rich biogenic material (such as radiolarian or sponge remains) that recrystallized within marine limestone or chalk, commonly during the Cretaceous and other sedimentary periods. Exact age and locality cannot be determined from the image.

Uses & applications

Historically used for sharp tools, arrowheads, and fire-starting; today it is chiefly a teaching, lapidary, and collecting material, with limited aggregate use.

Geological facts

Its conchoidal fracture produces exceptionally sharp edges, making it one of the most important prehistoric toolstones. Black color commonly reflects organic carbon or finely disseminated sulfides.

Field identification & locations

Identify it by its very hard, opaque fine texture, glassy sharp fracture, and inability to scratch it with a steel nail; it should not fizz with dilute acid unless coated in carbonate. Common in chalk and limestone regions, including Britain, France, Germany, and parts of North America.