Sedimentary

Flint pebble

Chert / Flint (SiO2)

Hardness 7 on the Mohs scale. Color ranges from grey to black, often with a white or lighter outer cortex. It has a waxy or dull luster and a cryptocrystalline structure. It exhibits conchoidal fracture.

Identified August 7, 2026More sedimentary →

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Physical properties

Formation & geological history

Flint is a form of cryptocrystalline quartz that typically occurs as nodules and masses in sedimentary rocks, such as chalks and limestones. It forms from the diagenesis of silica-rich marine sediments.

Uses & applications

Historically used for making stone tools and starting fires due to its hardness and ability to produce sparks. Today, it has minor uses in construction (as aggregate or building stone in some regions) and occasionally in lapidary.

Geological facts

Flint was crucial for early human technological development, used extensively during the Stone Age for tools and weapons.

Field identification & locations

Identify it by its hardness (scratches glass), waxy luster, and conchoidal fracture. Often found as rounded pebbles on beaches or in riverbeds where chalk or limestone deposits have eroded. The contrast between the dark interior and lighter rind is characteristic.