sedimentary

Chert (Black Chert or Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Dark grey to black
Luster: Waxy to dull

Identified March 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids within limestone or chalk beds. Often found in Paleozoic to Mesozoic sedimentary layers.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and 'flintlocks' for firearms. Modern uses include road construction aggregate and as a semi-precious gemstone when polished.

Geological facts

Black chert is often called 'Flint' when found in chalk or marly limestone. It was the primary material for the 'Stone Age' technological revolution due to its ability to hold a razor-sharp edge.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture and its ability to scratch glass easily. Commonly found along riverbeds, beaches, and eroded limestone outcrops. Collectors should look for sharp edges and a waxy texture.

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