Chert (often called Flint)

Physical Properties

Hardness: 7 (Mohs scale); Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & Geological History

Formed by the precipitation of silica from groundwater or within seafloor sediments, often replacing organic matter or calcium carbonate. Many specimens date to the Paleozoic or Mesozoic eras.

Uses & Applications

Historically used for stone tools (knapping) and fire-starting; currently used in construction aggregate, road surfacing, and as a semi-precious gemstone in jewelry.

Geological Facts

Flint is a variety of chert that occurs in chalk or marly limestone. It was the primary material for spearheads and arrowheads in prehistoric times due to its predictable conchoidal fracturing.

Field Identification & Locations

Identify by its waxy appearance, extreme hardness (scratches glass), and shell-like (conchoidal) curved fracture patterns. Commonly found in limestone riverbeds or gravel deposits.

Identified on: 5/1/2026

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