sedimentary

Flint or dark chert

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

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; dark gray-brown, opaque, waxy to dull luster, very fine-grained cryptocrystalline structure, and no visible cleavage. It shows conchoidal fracture with sharp edges; the rusty patches likely reflect iron-oxide staining or included host material.

Identified September 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms by replacement or precipitation of silica in marine sedimentary rocks, especially limestone and chalk, commonly during diagenesis. The specimen’s precise geological age cannot be determined visually; chert occurs from Precambrian rocks through modern formations.

Uses & applications

Used historically for flint tools and fire-starting, and today as a lapidary, teaching, and decorative material; generally not used as a major construction stone.

Geological facts

The sharp conchoidal fracture and glassy-looking break are characteristic of flint/chert. Flint is commonly considered a dark, high-quality variety of chert, often associated with chalk deposits.

Field identification & locations

Identify by testing for quartz-like hardness, inability to scratch it with a steel blade, and conchoidal fracture; it should scratch glass. Common in limestone, chalk, river gravels, and glacial deposits worldwide.