Chert (Jasper/Flint variant)

Physical Properties

Hardness: 7 (Mohs scale); Color: Brown, tan, and grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & Geological History

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation from silica-rich fluids within limestone or chalk beds. Most specimens can range from Precambrian to recent geological ages.

Uses & Applications

Historically used for stone tools and starting fires (flint). Currently used in jewelry (as jasper), as a construction aggregate, and as a popular specimen for amateur rock collectors.

Geological Facts

Chert is so hard it can scratch steel. Because it breaks with a conchoidal (shell-like) fracture, humans have used it for at least 2 million years to make sharp arrowheads, scrapers, and knife blades.

Field Identification & Locations

Identify in the field by its extreme hardness, waxy texture, and sharp, curved edges where broken. Commonly found in sedimentary basins, riverbeds, and as nodules within limestone or chalk outcrops.

Identified on: 4/25/2026

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