sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7; Color: Tan, brown, grey, or white with waxy appearance; Luster: Vitreous to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7
Color: Tan, brown, grey, or white with waxy appearance
Luster: Vitreous to waxy

Identified June 14, 2026More sedimentary →

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Physical properties

Hardness: 7; Color: Tan, brown, grey, or white with waxy appearance; Luster: Vitreous to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed by the accumulation of siliceous remains of organisms like diatoms or radiolarians, or via chemical precipitation from silica-rich groundwater replacing other minerals. Often found in limestone or chalk beds.

Uses & applications

Historically used for stone tools (arrowheads). Today used in road construction as aggregate, for lapidary work, and historically as fire-starter 'flint'.

Geological facts

Chert has a conchoidal fracture which produces a razor-sharp edge. It is one of the most common materials used by prehistoric humans for tool-making.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by steel) and curved, glass-like fracture surfaces. Often found as nodules in sedimentary basins or as river cobbles in gravel beds.