sedimentary

Chert (and Flint)

Silicon dioxide (SiO2) - Microcrystalline or Cryptocrystalline Quartz

AI-generated identification · may be incorrect

Hardness: 7 (Mohs scale); Color: Gray, black, tan, or brown; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Gray, black, tan, or brown
Luster: Waxy to dull

Identified March 22, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray, black, tan, or brown; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from hydrothermal fluids in sedimentary environments. Common in limestone and chalk beds.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to sharp edges; used in construction as road metal; sometimes polished as decorative stones or used as fire-starters (flint stones).

Geological facts

Chert was one of the most important materials in human prehistory for creating sharp-edged tools. The 'flintlock' mechanism in early firearms relied on flint's ability to create sparks when struck against steel.

Field identification & locations

Identify by its extremely smooth, shell-like (conchoidal) fracture and its ability to scratch glass. It is commonly found as nodules/concretions in limestone outcrops or as rounded river pebbles.