sedimentary

Chert (Flint)

Chert (Silicon Dioxide, SiO2)

Hardness: 7 on Mohs scale. Color: Gray, black, or blue-gray. Luster: Waxy to dull. Crystal structure: Cryptocrystalline (microscopic quartz crystals). Cleavage: None, exhibits conchoidal (shell-like) fracture. Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Gray, black, or blue-gray
Luster: Waxy to dull

Identified March 9, 2026More sedimentary →

Identify your own rocks.

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

App Store Google Play

Physical properties

Formation & geological history

Formed by the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like diatoms or radiolarians) in marine environments. Often found as nodules in limestone or chalk beds. Geological age can range from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges when fractured and used to start fires. Modernly used as a decorative stone, in construction aggregates, and for lapidary work.

Geological facts

Darker varieties are specifically called Flint. Because it breaks with a conchoidal fracture, it creates edges thinner than a steel scalpel. It was one of the most important minerals to prehistoric humans for survival.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), waxy appearance, and the way it breaks into curved, sharp flakes. Commonly found in limestone outcrops, riverbeds, and as beach pebbles in areas like the UK or the American Midwest.