Chert (Black Chert/Flint)

Physical Properties

Hardness: 7 (Mohs scale), Color: Dark grey to black, Luster: Waxy to dull, Crystal Structure: Cryptocrystalline (microscopic quartz), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & Geological History

Formed from the accumulation of siliceous remains of organisms like diatoms and radiolarians, or via chemical precipitation in sedimentary basins. Most commonly found in limestone and chalk beds.

Uses & Applications

Historically used for stone tools and starting fires (flint). Currently used as road aggregate, in jewelry (knapped art), and as a component in ceramics.

Geological Facts

Black chert is often specifically called flint. It was one of the most important minerals for early human technological development due to its ability to produce a sharp edge and sparks when struck against steel.

Field Identification & Locations

Identify in the field by its very high hardness (scratches glass), smooth conchoidal (shell-like) fracture edges, and waxy texture. Frequently found as nodules within carbonate rocks like limestone.

Identified on: 4/19/2026

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