Chert (often called Flint or River Rock)

Physical Properties

Hardness: 7 (Mohs scale); Color: Tan, brown, or gray; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like radiolarians or sponges) or by the chemical replacement of limestone/chalk by silica-rich fluids. Can be found in geological formations dating from the Precambrian to the present.

Uses & Applications

Historically used for stone tools and weapons due to sharp edges; currently used as road aggregate, architectural stone, and occasionally as a tumbling stone for hobbyists.

Geological Facts

Chert is so hard it will scratch glass and steel. High-quality varieties like Flint have been used for millennia to start fires because they create sparks when struck against steel.

Field Identification & Locations

Identify in the field by its smooth, waxy feel and shell-like (conchoidal) fracture pattern. Commonly found in stream beds or as nodules within limestone outcrops.

Identified on: 4/29/2026

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