sedimentary

Chert / Flint

Chert (SiO2)

Hardness: 6.5-7 Mohs; Color: Gray, tan, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8

Hardness: 6
Color: Gray, tan, or black
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 Mohs; Color: Gray, tan, or black; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.5-2.8

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from groundwater within limestone or chalk beds. Found in geological formations ranging from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools and weapons (arrowheads). Modern uses include road surfacing, jewelry as a semi-precious gemstone (Jasper), and fire-starting kits.

Geological facts

Flint is a dark variety of chert. When struck against steel, it produces sparks due to the friction oxidizing iron particles. It was one of the most important commodities in the prehistoric world.

Field identification & locations

Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Often found as nodules within limestone or as river pebbles.