sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, black, and tan with iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, black, and tan with iron staining
Luster: Waxy to dull

Identified May 13, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, black, and tan with iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed primarily in marine environments where microcrystalline silica precipitates from water or replaces organic material. It is found in many geological ages, often within limestone or shale beds.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) and fire-starting. Currently used in road construction, landscaping, and occasionally as jewelry when colorful (jasper).

Geological facts

Flint was essential to early human survival for making sharp blades and starting fires. It is so tough it can produce a spark when struck against steel.

Field identification & locations

Identified by its extremely smooth, shell-like (conchoidal) fracture surfaces and sharp edges. Found globally, especially in chalk or limestone deposits.