sedimentary

Flint or chert

Microcrystalline quartz (SiO₂), commonly chalcedony with possible minor impurities

AI-generated identification · may be incorrect

Usually Mohs 6.5–7; dark gray to brown, opaque, with a waxy to dull luster, very fine cryptocrystalline structure, and no visible cleavage. It fractures conchoidally, producing sharp edges; the pictured piece may be a naturally weathered or worked fragment.

Identified September 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces or fills cavities in limestone and other sediments, often during the Paleozoic–Cenozoic eras. It commonly occurs as nodules, bands, or layers in carbonate rocks.

Uses & applications

Used historically for stone tools and fire-starting, and today for decorative objects, lapidary work, aggregate, and occasional jewelry. Its sharp fracture makes it valuable archaeologically.

Geological facts

Flint is generally a dark, impure variety of chert colored by carbon, iron, or organic matter. Freshly fractured surfaces are typically smoother and glossier than weathered exteriors.

Field identification & locations

Look for a very hard, dense, fine-grained surface that scratches glass and breaks with curved, shell-like fractures; it commonly occurs in chalk and limestone regions worldwide, including England, France, Texas, and the Great Lakes.