Chert (Jasper/Black Chert)

Physical Properties

Hardness: 7 (Mohs scale), Color: Dark grey to black with brown weathering rind, Luster: Waxy to dull, Crystal structure: Microcrystalline/Cryptocrystalline (trigonal), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in deep-sea or sedimentary environments. Most specimens range from Precambrian to Cenozoic eras.

Uses & Applications

Historically used for stone tools/weapons (arrowheads); currently used in construction aggregate, landscaping, lapidary arts (tumbled stones), and as a silica source in glassmaking.

Geological Facts

Chert is a variety of quartz that was the 'high-tech' material of the Stone Age because it fractures consistently into sharp edges. Black chert is often called 'flint' if it occurs in chalk or limestone beds.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture pattern. Commonly found in riverbeds, glacial deposits, and as nodules within limestone or dolomite cliffs.

Identified on: 4/19/2026

Mode: Standard