sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, tan, or brown with a white weathered cortex; Luster: Waxy to vitreous; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7.

Hardness: 7 on Mohs scale
Color: Gray, tan, or brown with a white weathered cortex
Luster: Waxy to vitreous

Identified May 24, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily through the replacement of calcium carbonate by silica in marine environments, often appearing as nodules within limestone or chalk. Many specimens date from the Cretaceous period.

Uses & applications

Historically used for stone tools and weapons. Today used as an abrasive, in road construction, and as decorative gravel. It is also a popular specimen for beginner flint knapping.

Geological facts

Flint is a variety of chert that occurs in chalk or marly limestone. When struck against steel, flint produces sparks, making it essential for historic fire-starting and flintlock firearms.

Field identification & locations

Identifiable by its sharp, curved (conchoidal) edges and waxy interior hidden beneath a rough, chalky outer rind (cortex). Commonly found in lime-rich river beds and coastal chalk cliffs.