sedimentary

Chert (Flint)

Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 7 on Mohs scale
Color: Gray, black, or tan
Luster: Waxy to vitreous

Identified June 15, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, black, or tan; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Formation & geological history

Formed primarily in marine environments through the precipitation of silica from seawater or the accumulation of silica-rich organic remains like sponges and diatoms. It can also form as nodules in limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, knives) due to its sharp edges. Modernly used as a component in concrete, road surfacing, and as strike-stones for flintlock firearms or fire-starting kits.

Geological facts

Flint is a variety of chert. When struck against steel, it can produce sparks, which made it essential for early firearms and fire-starting throughout human history.

Field identification & locations

Identify in the field by its characteristic shell-like (conchoidal) fracture pattern and its ability to scratch glass. It is commonly found in areas with limestone or chalk deposits like the White Cliffs of Dover or the central United States.