sedimentary

Chert (specifically Dark Chert or Flint)

Silicon dioxide (SiO2)

AI-generated identification · may be incorrect

Hardness: 6.5-7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 6
Color: Dark grey to black
Luster: Waxy to dull

Identified June 28, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed by the accumulation of microcrystalline silica, often from the skeletal remains of silica-secreting organisms (like diatoms or radiolaria) in deep-sea environments, or through chemical precipitation in limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons due to its sharp edges when fractured. Modernly used in road construction, as a component of concrete, and sometimes as a specimen for geological study.

Geological facts

Dark-colored chert is often called flint. It was one of the most important minerals for prehistoric humans because it produces very sharp edges and can generate sparks for fire when struck against steel.

Field identification & locations

Identify it by its waxy luster, extreme hardness (it will scratch glass), and conchoidal (shell-like) fracture patterns. Frequently found in riverbeds, beaches, or as nodules in limestone outcrops.