Chert (specifically Flint or Common Chert)

Physical Properties

Hardness: 7 on Mohs scale; Color: Tan, beige, grey, or white; Luster: Dull to waxy; Crystal structure: Trigonal (microscopic); 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 groundwater within limestone or chalk beds. Most specimens found today date from the Paleozoic to Mesozoic eras.

Uses & Applications

Historically used for stone tools (arrowheads, scrapers) and starting fires (flint and steel). Currently used in road construction, as an abrasive, and by lapidary hobbyists for tumbling.

Geological Facts

Chert is so hard it can scratch glass and steel. Because it breaks with sharp, curved edges (conchoidal fracture), it was the primary material for prehistoric survival technologies across the globe.

Field Identification & Locations

Identify in the field by its distinct smooth, waxy fracture surfaces compared to a rough exterior. Look for it as nodules or layers within limestone. It will produce sparks when struck against high-carbon steel.

Identified on: 4/26/2026

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