sedimentary

Chert (Flint)

Microcrystalline silica (SiO2)

Hardness: 7; Color: Gray, tan, or brown with iron staining; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); SG: 2.65.

Hardness: 7
Color: Gray, tan, or brown with iron staining
Luster: Waxy to dull

Identified May 21, 2026More sedimentary →

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Physical properties

Hardness: 7; Color: Gray, tan, or brown with iron staining; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); SG: 2.65.

Formation & geological history

Forms through the precipitation of silica from groundwater filling pores in sedimentary rocks or through the accumulation of siliceous organic remains in marine environments.

Uses & applications

Historical use for stone tools/weapons due to conchoidal fracture; modern use in construction aggregate, road surfacing, and as a sparking agent (flint).

Geological facts

Chert was one of the most important materials for early humans because its sharp edges were ideal for making arrowheads and scrapers. It is resistant to chemical weathering.

Field identification & locations

Identify by its extreme hardness (scratches glass) and smooth, curved shell-like fracture surfaces. Often found as nodules within limestone or chalk beds.