Chert (and Flint)

Physical Properties

Hardness: 7 on Mohs scale; Color: dirty white to light tan; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none, breaks with conchoidal (shell-like) fracture.

Formation & Geological History

Formed from the chemical precipitate of silica or the accumulation of silica-rich microfossils like diatoms and radiolarians on the ocean floor. Commonly found in nodular or layered form within limestone or chalk beds.

Uses & Applications

Historically used for making stone tools (arrowheads, scrapers) and starting fires. Modernly used in construction aggregate and as a semi-precious gemstone when colorful (Jasper).

Geological Facts

Chert is so hard it can scratch steel. It was one of the most vital resources for prehistoric humans due to its ability to produce razor-sharp edges when struck.

Field Identification & Locations

Identify by its extreme hardness, waxy texture, and sharp, curved edges where it has broken. Look for it in limestone outcrops or as rounded pebbles in riverbeds.

Identified on: 5/15/2026

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