sedimentary

Chert / Flint

Silicon dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: typically dark gray, black, or brown (flint variety); Luster: waxy or dull; Crystal structure: cryptocrystalline; Cleavage: none, exhibits strong conchoidal fracture.

Hardness: 7 on Mohs scale
Color: typically dark gray, black, or brown (flint variety)
Luster: waxy or dull

Identified May 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within sedimentary host rocks like limestone or chalk.

Uses & applications

Historically used for stone tools and fire-starting; currently used for road surfacing, grinding media, and occasionally in jewelry as a gemstone.

Geological facts

Flint was one of the most important resources for prehistoric humans because it breaks into very sharp edges that could be used for arrowheads and knives.

Field identification & locations

Identify by its waxy luster, extreme hardness (scratches glass), and shell-like (conchoidal) curves on broken surfaces. Often found as nodules within chalk or limestone formations.