Chert (Black Flint)

Physical Properties

Hardness: 7 Mohs; Color: Black, dark gray with blue-tinted weathering; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed by the accumulation of siliceous remains of organisms (radiolarians/diatoms) or chemical precipitation from sea water in limestone or chalk beds. Often Cretaceous in age.

Uses & Applications

Historically used for making stone tools (arrowheads). Today used in jewelry, road construction, and as flint for fire-starting.

Geological Facts

Flint is a variety of chert that occurs in chalk or marly limestone. When struck against steel, it produces sparks, which led to its historic use in flintlock firearms.

Field Identification & Locations

Identify by its extremely smooth, waxy surface and circular (conchoidal) breakage patterns. Often found in riverbeds or coastal chalk cliffs. Collectors look for pieces with unique banding or high polish.

Identified on: 5/2/2026

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