Chert (and/or Flint)

Physical Properties

Hardness: 6.5-7 (Mohs scale). Color: Gray, white, or brown with a waxy luster. Crystal structure: Cryptocrystalline. Cleavage: None, exhibits sharp conchoidal (shell-like) fracture.

Formation & Geological History

Formed as nodules or layered deposits in limestone or chalk through the accumulation of silica-rich microfossils like diatoms or radiolarians. Most common in ancient marine sedimentary sequences.

Uses & Applications

Historically used for stone tools (arrowheads, knives) due to sharp edges and in fire-starting. Currently used as road metal, in decorative landscaping, or as concrete aggregate.

Geological Facts

Because of its extreme hardness and tendency to fracture with razor-sharp edges, chert was the primary material for technologies in the Stone Age. It can strike sparks against steel to ignite tinder.

Field Identification & Locations

Identify in the field by its waxy appearance and distinct conchoidal fracture which leaves curved pits. It is very hard and cannot be scratched by a steel knife.

Identified on: 5/19/2026

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