sedimentary

Polished brown flint/chert nodule

Microcrystalline quartz (SiO₂), commonly called flint or chert

AI-generated identification · may be incorrect

Hardness about 7 Mohs; dense, opaque brown to black with waxy-to-glassy luster and conchoidal fracture. The smooth surface appears naturally waterworn or lightly polished; visible dark and brown zones may reflect impurities and weathering.

Identified September 12, 2026More sedimentary →

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Physical properties

Formation & geological history

A chemical/biogenic sedimentary silica replacement formed within limestone or chalk, generally during the Cretaceous or younger sedimentary periods. Nodules develop as dissolved silica precipitates and replaces carbonate material, later exposed and rounded by erosion.

Uses & applications

Historically important for stone tools and fire-starting; today used mainly as a teaching, lapidary, or display specimen and occasionally as aggregate or decorative stone.

Geological facts

Flint and chert are mineralogically similar; “flint” commonly refers to dark chert associated with chalk. Conchoidal edges can be extremely sharp when freshly fractured.

Field identification & locations

Identify by its very high hardness, inability to scratch with a steel blade, opaque waxy appearance, and curved shell-like fracture. Common in chalk and limestone regions, including England, France, the Great Lakes, and many sedimentary basins.