Chert (often called Flint when dark)

Physical Properties

Hardness: 7 on Mohs scale; Color: Cream, white, Tan, or brown with possible banding; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (displays conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed from the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the seafloor, which then undergo diagenesis. It can also form as nodules in limestone through chemical replacement.

Uses & Applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; modern uses include road surfacing, gemstones, and as a component in glass manufacturing.

Geological Facts

Because of its high hardness and brittle nature, prehistoric humans used it as their primary material for tools and weapons. It can strike a spark against steel, making it vital for fire-starting throughout history.

Field Identification & Locations

Identify in the field by its smooth, shell-like (conchoidal) breaks that result in very sharp edges. It will not be scratched by a steel knife. Commonly found in limestone beds or as water-worn pebbles in stream beds.

Identified on: 4/17/2026

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