sedimentary

Chert (likely flint)

Microcrystalline quartz, SiO₂ (chalcedony-rich chert)

AI-generated identification · may be incorrect

Hardness 6.5–7; brown to olive-gray, waxy to dull luster, dense and opaque. It has no visible crystals or cleavage and commonly breaks with smooth conchoidal fractures; the pale crust may be weathered silica or cortex.

Identified August 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces carbonate sediment or when microscopic siliceous sediment lithifies, commonly in limestone or chalk. Most specimens are Paleozoic to Cenozoic, though chert occurs throughout the geologic record.

Uses & applications

Historically used for sharp tools, arrowheads, and fire-starting; today it is mainly a teaching or collecting specimen and occasionally used as aggregate or decorative stone.

Geological facts

Fresh edges can be extremely sharp because of conchoidal fracture. Flint is a dark, compact variety of chert, while color depends on iron oxides, organic matter, and other impurities.

Field identification & locations

Look for very high hardness, inability to scratch with a steel knife, waxy texture, and shell-like fracture; it commonly occurs as nodules or bands in limestone. Common sources include England, France, the Great Lakes region, Texas, and many sedimentary basins.