Chert (Flint)

Physical Properties

Hardness: 7 (Mohs scale); Color: White, tan, or grey (often weathered); Luster: Waxy to dull; Crystal Structure: Microcrystalline/cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Formation & Geological History

Formed from the accumulation of siliceous remains of organisms like diatoms or radiolarians, or via chemical precipitation within limestone or chalk beds. Most specimens date from the Phanerozoic eon.

Uses & Applications

Historically used for stone tools and starting fires (striking steel). Modernly used in construction aggregate, road surfacing, and as a low-cost gemstone or garden stone.

Geological Facts

Chert is so hard it can scratch glass and steel. It was the primary material for spearheads and arrowheads for thousands of years due to its predictable conchoidal fracture which creates razor-sharp edges.

Field Identification & Locations

Identify by its extreme hardness, smooth waxy texture when broken, and curved (conchoidal) fracture lines. Commonly found as nodules or layers in limestone or chalk deposits globally.

Identified on: 5/1/2026

Mode: Standard