sedimentary

Flint

SiO2 (Microcrystalline Quartz / Chert)

Hardness of 7 on the Mohs scale. Colors vary from grey, white, brown, to dark blue-black, often with iron oxide staining (patina) on the outer cortex. Luster is vitreous to waxy. It has a cryptocrystalline crystal structure and exhibits conchoidal fracture with sharp edges, lacking true cleavage.

Identified August 5, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical replacement of calcium carbonate in limestone or chalk by silica-rich solutions derived from sponge spicules or other silica sources in marine environments, typically during the Cretaceous period.

Uses & applications

Historically used for making stone tools, arrowheads, and striking fire due to its conchoidal fracture and sparks produced when hit with steel. Today used for decorative landscaping, gravel, and occasionally lapidary work.

Geological facts

Flint is essentially the same mineral as chert, but the term 'flint' is traditionally reserved for varieties found as nodules in chalk or marl formations.

Field identification & locations

Easily identified in coastal or river environments by its smooth, water-worn exterior cortex, waxy interior fracture surface, and extreme hardness that scratches glass. Common globally in chalk-bearing geological formations such as the cliffs of Dover and beaches along the North Sea.