Chert (often called Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: Grey, tan, smoky; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & Geological History

Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. Often found as nodules in limestone or chalk beds.

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Modernly used in construction aggregate, road surfacing, and as a starting material for silicon.

Geological Facts

Because of its conchoidal fracture, chert breaks into edges that are sharper than a surgical steel scalpel. Native Americans and early humans worldwide prized it for toolmaking.

Field Identification & Locations

Identify by its 'waxy' appearance, extreme hardness (it will scratch glass), and its tendency to break in smooth, curved shell-like patterns known as conchoidal fractures. Commonly found in gravel pits or weathered out of limestone cliffs.

Identified on: 4/13/2026

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