sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 Mohs; Color: Black, dark gray, tan; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 Mohs
Color: Black, dark gray, tan
Luster: Waxy to vitreous

Identified July 19, 2026More sedimentary →

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Physical properties

Hardness: 7 Mohs; Color: Black, dark gray, tan; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed as nodules or beds in sedimentary rocks (like limestone or chalk) through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains.

Uses & applications

Historically used for making stone tools (arrowheads, knives) and for starting fires; currently used as road aggregate or in jewelry (lapidary).

Geological facts

Flint was one of the most important minerals for early humans because of its ability to fracture with extremely sharp edges. It sparks when struck against steel, which is unique among most common rocks.

Field identification & locations

Identify by its waxy appearance, sharp edges, and characteristic 'scoop-like' shell-shaped (conchoidal) fractures. Often found in chalk cliffs or as stream-worn pebbles.