sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.

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

Identified June 30, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.

Formation & geological history

Formed as nodules or beds in sedimentary rocks like limestone or chalk through the accumulation of silica-rich organic remains (like sponge spicules or radiolarians).

Uses & applications

Historically used for stone tools and starting fires; currently used in construction aggregate, road surfacing, and as a semi-precious gemstone.

Geological facts

Chert was one of the most important minerals for early humans because its conchoidal fracture creates razor-sharp edges used for arrows, spears, and knives.

Field identification & locations

Identify in the field by its very smooth, waxy surface and sharp, curved (conchoidal) edges when broken. It will easily scratch glass and does not react with acid.