Chert (often called Flint when dark)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray, tan, white, and bluish-gray; Luster: Waxy to dull; Structure: Cryptocrystalline; 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 through the chemical replacement of limestone/dolomite by silica-rich fluids. Can be billions of years old or relatively recent.

Uses & Applications

Used historically for stone tools and starting fires (striking steel). Modern uses include road surfacing, jewelry (when colorful as jasper or agate), and as an abrasive.

Geological Facts

Chert is so hard it can scratch glass easily. Its conchoidal fracture creates razor-sharp edges, which is why it was the primary material for arrowheads and knives for prehistoric humans.

Field Identification & Locations

Identify in the field by its hardness (won't be scratched by a steel knife) and its smooth, shell-like (conchoidal) breakage patterns. Common in limestone beds or as rounded stream pebbles.

Identified on: 4/26/2026

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