sedimentary

Chert (varieties include Flint and Jasper)

Microcrystalline or Cryptocrystalline Silicon Dioxide (SiO2)

Hardness: 7 on the Mohs scale; Color: Gray, brown, or tan (due to organic material/clay impurities); Luster: Dull to waxy; Structure: Cryptocrystalline quartz; Cleavage: None (conchoidal fracture); Specific Gravity: 2.5–2.8.

Hardness: 7 on the Mohs scale
Luster: Dull to waxy

Identified March 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation during diagenesis in marine environments. It occurs as nodules or layered beds within limestone and chalk.

Uses & applications

Historically used for stone tools (knapping for spearheads/arrowheads) and as fire-starters. Modern uses include road surfacing material, decorative garden stones, and occasionally as a base for polishing into cabochons for jewelry.

Geological facts

Chert is so hard that it can scratch steel and was a primary survival resource for prehistoric humans. The variety 'Flint' specifically refers to chert found in chalk or marly limestone formations.

Field identification & locations

Identify in the field by its distinct conchoidal (shell-like) fracture pattern with sharp edges and its ability to scratch glass. Commonly found in riverbeds, glacial tills, and eroded limestone outcrops. Found globally, especially in the central and eastern United States.

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