sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs); Color: Dark gray to black with tan/brown cortex; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 (Mohs)
Color: Dark gray to black with tan/brown cortex
Luster: Waxy to vitreous

Identified July 7, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs); Color: Dark gray to black with tan/brown cortex; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed in marine or lake environments as nodules within limestone or chalk through the accumulation of silica-rich organisms (like radiolarians or sponges) or by chemical precipitation. Found worldwide in sedimentary layers.

Uses & applications

Historically used for stone tools due to sharp edges; modern uses include decorative gravel, jewelry (cabochons), and as a basic fire-starting tool when struck against steel.

Geological facts

Flint is a variety of chert that has been prized by humans for over a million years for its predictable conchoidal fracture, which allows for the creation of sharp blades and arrowheads.

Field identification & locations

Identified by its extreme hardness, waxy surface, and characteristic curved, shell-like (conchoidal) fracture pattern. Found frequently as beach pebbles or in sedimentary outcrops.