sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs; Color: Black, dark gray with white cortex; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 Mohs
Color: Black, dark gray with white cortex
Luster: Waxy to dull

Identified July 11, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Black, dark gray with white cortex; 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 accumulation of silica-rich organic remains or chemical precipitation within limestone or chalk beds. This specimen features a typical weathered white chalky outer layer (cortex).

Uses & applications

Historically used for stone tools (knapping) and strike-a-lights for fire making. Modernly used as road surfacing, decorative gravel, and occasionally in jewelry as cabochons.

Geological facts

Flint was one of the first materials traded by early humans. When struck against steel, it creates sparks due to the friction scraping off tiny particles of iron that spontaneously ignite.

Field identification & locations

Identify by its smooth, shell-like (conchoidal) fracture surfaces and sharp edges. It is harder than steel and will easily scratch glass. Commonly found as nodules in chalk or limestone cliffs.