sedimentary

Chert (Flint)

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: Brown, dark grey, or black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Brown, dark grey, or black
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolaria) or via chemical precipitation from silica-rich fluids within limestone or chalk beds. Often found in nodules.

Uses & applications

Historically used for stone tools and weapons (arrowheads); currently used in road construction, as a medium for glass production, and occasionally in jewelry as Jasper.

Geological facts

Flint is a variety of chert that was essential to prehistoric humans for fire-starting due to the sparks produced when struck against steel or pyrite. It breaks with extremely sharp edges, sharper than a surgeon's scalpel.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Commonly found in sedimentary basins, especially within limestone layers or as stream pebbles.