sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 (Mohs scale), Color: transitions from dark grey/brown to tan/white (outer cortex), Luster: waxy to dull, Crystal structure: cryptocrystalline, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6

Hardness: 6

Identified June 3, 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 through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine environments. This specimen shows a transition between the dark interior chert and the weathered white outer rind (cortex).

Uses & applications

Historically used for stone tools and weapons due to sharp edges when fractured; modern uses include road construction aggregate, fire-starting strikers, and as a polishing medium.

Geological facts

Chert was one of the most vital materials for early humans because its conchoidal fracture allows it to be knapped into extremely sharp blades. The white coating seen on one side is known as 'patina' or 'cortex' caused by weathering and chemical leaching.

Field identification & locations

Identify by its 'toffee-like' smoothness, conchoidal (shell-like) fracture patterns, and its ability to scratch glass. Commonly found in limestone beds or as water-worn pebbles in riverbeds and on beaches.