Chert (often called Flint for dark varieties)

Physical Properties

Hardness: 7 (Mohs scale), Color: Varying shades of gray, brown, and green; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich microfossils or via chemical precipitation from silica-rich groundwater over millions of years, often as nodules in limestone or chalk beds.

Uses & Applications

Historically used for stone tools (knapping) and starting fires. Modern use includes road surfacing, grinding media, and occasionally as a decorative semi-precious stone.

Geological Facts

Because it breaks with a conchoidal (shell-like) fracture, it produces incredibly sharp edges. It has been used for over 2 million years to manufacture scrapers, arrowheads, and spear points.

Field Identification & Locations

Identify in the field by its waxy texture and sharp, curved edges when broken. It cannot be scratched by a steel pocket knife. Commonly found in limestone outcrops or as smooth pebbles in riverbeds.

Identified on: 4/14/2026

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