sedimentary

Flint (Chert)

Cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Brown, grey, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Brown, grey, tan
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Brown, grey, tan; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed as nodules or masses in sedimentary rocks like chalk and limestone. Created via the replacement of organic material or the precipitation of silica from groundwater. Often Cretaceous or Jurassic in age.

Uses & applications

Historically used for stone tools and fire-starting. Currently used in road construction, ceramics, and as a polishing media in ball mills.

Geological facts

Flint was the primary material for the Stone Age tool industry due to its predictable conchoidal fracture which creates sharp edges. It produces sparks when struck against steel.

Field identification & locations

Identify by its waxy texture, smooth conchoidal (shell-like) fracture surfaces, and extreme hardness. Commonly found in coastal chalk cliffs and riverbeds containing eroded limestone.