sedimentary

Chert (Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Grey, tan, brown, or black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 7 on Mohs scale
Color: Grey, tan, brown, or black
Luster: Waxy to dull

Identified June 14, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Grey, tan, brown, or black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & geological history

Formed from the accumulation of silica-rich microfossils (like radiolaria or diatoms) or by chemical precipitation from groundwaters in limestone or chalk beds. Commonly found in Mesozoic or Paleozoic strata.

Uses & applications

Historical use for stone tools (knapping), fire-starting (flint and steel), road ballast, and decorative garden stones. High-quality varieties like Jasper are used in jewelry.

Geological facts

Chert is so hard it can scratch steel. It was the primary material used by prehistoric humans to create arrowheads, scrapers, and axe heads due to its predictable conchoidal fracturing.

Field identification & locations

Identify by its 'shell-like' conchoidal fracture patterns and waxy texture. It is often found as nodules within limestone or chalk cliffs or as stream-worn cobbles.