sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: typically gray, black, or brown with a lighter weathered cortex; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Luster: waxy to dull

Identified July 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical replacement of limestone/chalk by silica-rich fluids. Often found as nodules or layers in sedimentary rock beds from various geological eras.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges; used in construction aggregate and as decorative landscaping stones.

Geological facts

Flint is a variety of chert that was the first 'mineral' mined during the Stone Age. When struck against steel, it creates sparks, which made it essential for early firearms (flintlocks) and fire-starting.

Field identification & locations

Identify in the field by its smooth, waxy fracture surfaces, extreme hardness (it will scratch glass and steel), and the way it breaks into sharp, curved 'shell-like' edges. Often found as rounded nodules within lighter chalk or limestone cliffs.