sedimentary

Chert (Flint)

Microcrystalline Silica (SiO2)

Hardness: 7 (Mohs scale), Color: tan, grey, or brownish-grey, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6

Identified July 18, 2026More sedimentary →

Explore Chert (Flint) in the encyclopedia →

Identify your own rocks.

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

App Store Google Play

Physical properties

Hardness: 7 (Mohs scale), Color: tan, grey, or brownish-grey, Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6

Formation & geological history

Formed as nodules or beds in sedimentary rocks, typically through the accumulation of silica-rich organic remains or chemical precipitation from groundwater during the Paleozoic or Mesozoic eras.

Uses & applications

Historically used for stone tools like arrowheads and flintlock firearms. Currently used in road construction, as an abrasive, and sometimes for decorative landscape gravel.

Geological facts

Chert breaks with a conchoidal fracture, creating very sharp edges, which made it the primary material for tool-making in the Stone Age. Flint is a specific variety of dark chert found in chalk or marly limestone.

Field identification & locations

Identify in the field by its waxy appearance, extreme hardness (it will scratch glass), and the way it breaks into curved, shell-like flakes. Common in limestone outcrops and riverbeds.