sedimentary

Flint/chert nodule with a weathered cortex

Microcrystalline quartz (SiO₂), likely flint or dark chert

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; black to gray, opaque, with a dull-to-waxy luster and conchoidal fracture. The tan, rough outer surface is weathered cortex, while the dark interior is cryptocrystalline quartz.

Identified September 13, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater replaces or fills cavities in limestone, chalk, or other sediments; the specimen likely formed during the host sediment’s burial and diagenesis, commonly in Mesozoic–Cenozoic strata. Weathering exposed the nodule and altered its outer rind.

Uses & applications

Historically used for sharp tools, arrowheads, and fire-starting; today it is mainly collected, studied, or used decoratively. It is not typically a gemstone or construction material.

Geological facts

Freshly fractured flint produces extremely sharp edges and may strike sparks from steel. Its dark color commonly reflects organic matter, iron sulfides, or other impurities.

Field identification & locations

Identify it by its very hard, non-granular texture, waxy surface, and curved conchoidal breaks; it should scratch glass and resist a steel knife. Common in chalk and limestone regions, including England, France, Denmark, and parts of the United States.