Chert (often called Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Black, dark grey, or brownish (due to organic matter); Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation in marine environments where silica replaces carbonate minerals. Many specimens date back to the Cretaceous or older.

Uses & Applications

Historically used for stone tools (knapping) and starting fires (striking steel). Modern uses include road surfacing, jewelry (as lapidary material), and as an abrasive in some industrial processes.

Geological Facts

Because of its conchoidal fracture, it breaks into razor-sharp edges, making it the primary material for prehistoric arrowheads and spear points. Darker varieties are chemically almost identical to lighter chalcedony but contain more carbonaceous impurities.

Field Identification & Locations

Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its distinct shell-like (conchoidal) curved fracture patterns. Commonly found in limestone or chalk beds as nodules.

Identified on: 4/18/2026

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