Chert (Flint variety)

Physical Properties

Hardness: 6.5–7 (Mohs); Color: Dark grey to black with brownish weathering; Luster: Dull to waxy; Crystal structure: Microcrystalline/cryptocrystalline (concealed); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed from the accumulation of siliceous remains of marine organisms (like radiolarians or diatoms) or secondary chemical precipitation in limestone/chalk beds. Can range from Precambrian to Holocene in age.

Uses & Applications

Historically used for stone tools and fire-starting. Modern uses include aggregate in construction, road surfacing, and as a polishing media in rock tumblers.

Geological Facts

Flint was the most important raw material for early humans to create sharp-edged tools. When struck against steel or pyrite, it produces sparks, making it essential for early firearms (flintlocks).

Field Identification & Locations

Identify by its waxy texture and characteristic conchoidal (shell-like) fracture edges which are very sharp. Look for it in chalk or limestone outcrops. It will scratch glass easily.

Identified on: 4/18/2026

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