Chert (specifically Flint or Jasper-Chert)

Physical Properties

Hardness: 7 (Mohs scale); Color: Dark gray/black with tan/brown exterior; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the replacement of calcium carbonate by silica in sedimentary rock layers, often as nodules in limestone or chalk during the Diagenesis process. These materials range from the Precambrian to Cenozoic eras.

Uses & Applications

Historically used for stone tools (knapping) and fire-starting. Currently used as a gemstone (lapis-chert), in road aggregate, and for landscaping.

Geological Facts

Because of its conchoidal fracture (breaking like glass to create razor-sharp edges), flint was the primary technology for weapon and tool making for thousands of years in the Stone Age.

Field Identification & Locations

Identify by its 'conchoidal' (shell-like) fracture pattern and the ability to scratch glass. It often has a weathered, lighter-colored outer 'rind' or cortex, as seen in the specimen. Common in river beds and limestone outcrops.

Identified on: 4/15/2026

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