sedimentary

Chert (likely flint)

Microcrystalline quartz, SiO₂ (chalcedony/quartz aggregate)

AI-generated identification · may be incorrect

Dark brown to black, waxy-to-dull luster, very fine-grained and opaque with irregular conchoidal fracture; Mohs hardness about 6.5–7, no cleavage, specific gravity roughly 2.6. The pale gray patches may be quartz or weathered/chalcedonic zones.

Identified August 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms mainly by silica replacement of carbonate sediment or accumulation of microscopic silica in marine sediments, commonly during the Paleozoic–Mesozoic and younger periods. Weathering leaves resistant nodules and pebbles.

Uses & applications

Used historically for sharp tools and fire-starting, and today for decorative stone, aggregate, and lapidary work; attractive pieces have modest collecting value.

Geological facts

Flint is a dark, dense variety of chert; freshly broken surfaces are typically sharp-edged and glasslike. Trace iron and organic matter commonly produce brown-black coloration.

Field identification & locations

Identify by testing for glass-scratching hardness, low porosity, and conchoidal fracture; it commonly occurs as nodules in limestone or as stream-worn cobbles. Exact locality and pattern are needed for precise identification.