sedimentary

Chert (Black Flint)

Silicon Dioxide (SiO2)

Hardness: 7 Mohs; Color: Black to dark gray; Luster: Waxy to dull; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 Mohs
Color: Black to dark gray
Luster: Waxy to dull

Identified July 19, 2026More sedimentary →

Explore Chert (Black 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; Color: Black to dark gray; Luster: Waxy to dull; Structure: Cryptocrystalline/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7

Formation & geological history

Formed through the accumulation of silica-rich organic remains or chemical precipitation in deep-sea or limestone environments. It often occurs as nodules or layers within sedimentary rock beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) and starting fires (flint and steel). Currently used in construction aggregate and as a collector's specimen.

Geological facts

Black chert is often specifically called flint. Its ability to break with extremely sharp edges made it the primary technological material for early humans for hundreds of thousands of years.

Field identification & locations

Identify in the field by its hardness (scratches glass), smooth conchoidal (shell-like) fractures, and waxy texture. Commonly found in limestone outcrops or along riverbeds where erosion has released the nodules.