sedimentary

Chert (Flint)

Cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale), Color: White to light grey with brown staining, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified July 18, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: White to light grey with brown staining, Luster: Waxy to dull, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed as nodules or beds in sedimentary rocks through the precipitation of silica from groundwater or the accumulation of siliceous biological remains like diatoms or radiolaria. Found in limestone or chalk deposits.

Uses & applications

Historically used for making stone tools (knapping) and fire-starting. Currently used in road construction, as an abrasive, and in laboratory mortar and pestle sets.

Geological facts

Chert has been used by humans for hundreds of thousands of years to create arrowheads and scrapers because it breaks with incredibly sharp edges. It is often referred to as 'Flint' when found in chalk or marly limestone.

Field identification & locations

Identified by its extreme hardness (cannot be scratched by steel), waxy appearance, and characteristic shell-like (conchoidal) fracture patterns. Frequently found in riverbeds or eroded limestone cliffs.