sedimentary

Flint

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Dark grey to black with white exterior (cortex); Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None, exhibits conchoidal fracture (shell-like curves).

Hardness: 7 on Mohs scale
Color: Dark grey to black with white exterior (cortex)
Luster: Waxy to vitreous

Identified July 3, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in sedimentary rocks, typically within chalk or marly limestone beds. It originates from the accumulation of silica-rich skeletal remains of marine organisms like sponges and diatoms during the Cretaceous period.

Uses & applications

Historically used for making stone tools and starting fires; currently used in glass manufacturing, as an abrasive, and in construction/road building. Small polished pieces are collected for hobbyist interests.

Geological facts

Flint is a variety of chert. When struck against steel, it produces sparks, which led to its widespread use in flintlock firearms. The coordinate provided (South-East Ireland) is consistent with glacial deposits and coastal gravels where flint is abundant.

Field identification & locations

Look for a smooth, glass-like dark interior when broken and a chalky white outer layer. It is very hard and will scratch window glass. Common in glacial till and beaches across the British Isles and Ireland.