sedimentary

Chert (and/or Flint)

Chert (Cryptocrystalline variety of Quartz, SiO2)

Hardness: 7 (Mohs scale), Color: Varies from dark grey/black to light tan, Luster: Vitreous to waxy, Crystal Structure: Cryptocrystalline (microscopic), Cleavage: None (has conchoidal fracture), Specific Gravity: 2.65

Identified April 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine environments. This specimen shows distinct banding or laminations, common in nodules or beds within limestone and chalk.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modern uses include road base, concrete aggregate, and sometimes as a flux in blast furnaces. High-quality colorful varieties may be used in jewelry as cabochons.

Geological facts

Chert is so hard it can strike sparks against steel, which made it the primary material for flintlock firearms. The word 'flint' is often used interchangeably with dark chert found in chalk deposits.

Field identification & locations

Identified in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass easily. It often has a weathered white or tan 'rind' or cortex on the outside. Common worldwide in sedimentary basins.

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