sedimentary

Black Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Black with white quartz/calcite veining; Luster: Waxy to dull; Crystal Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (Conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Black with white quartz/calcite veining
Luster: Waxy to dull

Identified July 5, 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 in marine environments; commonly found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools and starting fires; currently used as aquarium decor, in landscaping, and as a source of silica in glass manufacturing.

Geological facts

Black chert is often called flint when found in chalk or marly limestone. Because of its conchoidal fracture, it breaks into extremely sharp edges, which is why it was the primary material for prehistoric arrowheads and knives.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by steel), waxy texture, and smooth, curved fracture surfaces. Commonly found in riverbeds or coastal areas near sedimentary basins.