sedimentary

Chert (Grey-Green Flint)

Microcrystalline silicon dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 on the Mohs scale. Color: Opaque grey or dull green. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (exhibits conchoidal fracture). Specific gravity: 2.65.

Hardness: 6
Color: Opaque grey or dull green
Luster: Waxy to dull

Identified March 26, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 on the Mohs scale. Color: Opaque grey or dull green. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (exhibits conchoidal fracture). Specific gravity: 2.65.

Formation & geological history

Formed from the chemical precipitation of silica-rich fluids or the accumulation of microscopic skeletal remains of silica-secreting organisms (like diatoms or radiolarians) in marine environments. It is often found as nodules in limestone or chalk beds. Geological age ranges from Precambrian to Cenozoic.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges when fractured. Modern uses include road surfacing, jewelry (when polished), and as a raw material for silicon production.

Geological facts

Chert has a unique property where it produces sparks when struck against steel, which made it essential for early firearms (flintlocks) and fire-starting. It is incredibly durable and chemically stable, often surviving billions of years of weathering.

Field identification & locations

Identify in the field by its characteristic shell-like (conchoidal) fracture pattern and its ability to scratch glass. It often feels smooth or waxy. Common in riverbeds, glacial deposits, and embedded in sedimentary rock layers.