sedimentary

Chert (Flint Nodular Pebble with Weathering Cortex)

Microcrystalline / Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Grayish-tan translucent core with an opaque white, chalky weathering cortex (patina); Luster: Waxy to dull/earthy on weathered surface, vitreous to waxy on fresh breaks; Cleavage: None (exhibits conchoidal fracture); Specific Gravity: 2.5 - 2.7.

Hardness: 7 on Mohs scale

Identified November 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the diagenetic precipitation of silica within marine carbonate sediments (such as limestone or chalk) during the Paleozoic, Mesozoic, or Cenozoic eras. The outer white layer is a porous cortex resulting from leaching and weathering of silica and remnant carbonate minerals.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) and generating sparks for fire-starting/flintlock firearms; currently used as construction aggregate, ballast, and in lapidary work.

Geological facts

Because chert breaks with extremely sharp conchoidal edges, it was one of the most critical materials in prehistoric human technology, rivaled only by obsidian. The white powdery surface is often referred to by collectors as 'patina' or 'chalk cortex'.

Field identification & locations

Identified by high hardness (scratches glass and steel), smooth water-worn rounded contours, conchoidal fractures peeking out where chipped, and a typical chalky white outer rind. Abundantly found on river gravel bars, marine beaches, and within chalk/limestone formations worldwide.