Chert (often called Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Gray to brownish-gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline/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 precipitation of silica in groundwater within limestone or chalk beds. Most common in Paleozoic or Mesozoic strata.

Uses & Applications

Historically used for stone tools and starting fires (striking steel). Modern uses include road surfacing, abrasives, and decorative landscaping.

Geological Facts

Chert was one of the most important minerals for early humans because its conchoidal fracture creates extremely sharp edges perfect for arrowheads and scrapers. 'Flint' is technically a dark-colored variety of chert.

Field Identification & Locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breaks. Often found as nodules within limestone or as river pebbles.

Identified on: 4/26/2026

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