sedimentary

Chert (and Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with a yellowish-orange weathering rind; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Hardness: 7 on Mohs scale
Color: Gray, tan, or brown with a yellowish-orange weathering rind
Luster: Waxy to dull

Identified July 2, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with a yellowish-orange weathering rind; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture).

Formation & geological history

Formed in sedimentary environments through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation in groundwater replacing limestone or chalk. Many specimens date from the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used in road construction, as an abrasive, and sometimes in jewelry as tumbled stones.

Geological facts

Chert is so hard it will scratch steel. It was one of the most vital materials for prehistoric humans because it breaks with incredibly sharp, glass-like edges suitable for arrowheads and scrapers.

Field identification & locations

Identify by its 'conchoidal' (shell-like) fracture patterns and ability to scratch glass. Often found as nodules within limestone or chalk beds, or as rounded pebbles in riverbeds.