sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray to dull green; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray to dull green
Luster: Waxy to vitreous

Identified June 3, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray to dull green; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Forms primarily through the deposition of silica-rich fluids in sedimentary environments. Often found as nodules in limestone or chalk beds, it can also form as bedded chert from the accumulation of microscopic silica-shelled organisms like radiolarians or diatoms.

Uses & applications

Historically used for stone tools and weapons (flintknapping); currently used in road construction as aggregate, as a component in ceramics, and occasionally for lapidary art or fire-starting kits.

Geological facts

Chert is so tough and durable that it was the primary material for human tools for millions of years. The distinctive spark produced when struck against steel is due to the hardness of the silica.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture and its ability to scratch glass. It is commonly found in gravel pits, riverbeds, and weathered out of limestone cliffs.