sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Black, dark gray, or olive brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Black, dark gray, or olive brown
Luster: Waxy to dull

Identified July 5, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Black, dark gray, or olive brown; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed in marine environments through the precipitation of silica from seawater or the accumulation of siliceous skeletal remains (like radiolarians or diatoms) that recrystallize within limestone or chalk beds.

Uses & applications

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

Geological facts

Flint is a variety of chert that occurs in chalk or marly limestone. It was the primary material for prehistoric technology for over two million years due to its predictable conchoidal fracturing which produces razor-sharp edges.

Field identification & locations

Identify by its smooth, waxy surface and shell-like (conchoidal) fractures. It will easily scratch glass and can produce sparks when struck against steel. Common in chalk cliffs and riverbeds.