sedimentary

Flint Nodule with Chalk Cortex

Chert (Microcrystalline Quartz) SiO2

AI-generated identification · may be incorrect

Hardness: 7 (Mohs), Color: Dark grey to black internally, with a white/yellowish porous outer cortex, Luster: Waxy to dull, Crystal structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific gravity: 2.6

Identified August 31, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in marine environments as nodules within chalk deposits. Often Cretaceous in age. Silica derived from sponge spicules and microscopic marine organisms precipitated and replaced calcium carbonate.

Uses & applications

Historically used for making stone tools (arrowheads, hand axes) due to its conchoidal fracture, and for striking sparks to start fires or in flintlock firearms. Currently used occasionally in construction or as a minor decorative stone.

Geological facts

Flint was crucial for early human technological development during the Stone Age. The white chalky layer on the outside is called the 'cortex'.

Field identification & locations

Identified by its very hard, fine-grained, glassy to waxy interior, often dark colored, contrasted with a softer, white chalky rind. Found on beaches, in fields, and in chalk pits in regions like southern England and parts of Europe.