sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Variegated gray, tan, and black; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Hardness: 6
Color: Variegated gray, tan, and black
Luster: Dull to waxy

Identified June 14, 2026More sedimentary →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Variegated gray, tan, and black; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or through the chemical precipitation of silica from groundwater in limestone or chalk beds.

Uses & applications

Historically used for stone tools and starting fires; currently used in construction aggregate, road surfacing, and as a polishing abrasive.

Geological facts

Chert was one of the first materials used by early humans for tools because it breaks into sharp edges. It is chemically very stable and often preserves fossils in exceptional detail.

Field identification & locations

Identify in the field by its very hard surface (can scratch glass) and smooth, shell-like (conchoidal) fracture pattern. Often found as nodules within limestone or as river pebbles.