sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Grey, black, or brown; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 on Mohs scale
Color: Grey, black, or brown
Luster: Waxy to dull

Identified May 9, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Grey, black, or brown; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & geological history

Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the ocean floor or via chemical precipitation in limestone/chalk beds. Often found in nodules within Cretaceous chalk deposits.

Uses & applications

Historically used for making stone tools (arrowheads, knives) and starting fires. Today used as a decorative stone in landscaping, road construction aggregate, and occasionally in jewelry as cabochons.

Geological facts

Flint was the primary material for the Stone Age technology due to its predictable conchoidal fracture which creates razor-sharp edges. It produces sparks when struck against steel, a property that led to the flintlock mechanism in early firearms.

Field identification & locations

Identify by its waxy texture, extreme hardness (it will scratch glass), and the shell-like (conchoidal) curves where it has broken. Commonly found in chalk or limestone cliffs and on gravel beaches in the UK and Northern Europe.