sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale), Color: Dark grey to black, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified July 21, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: Dark grey to black, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Formation & geological history

Formed as nodules in sedimentary rocks like chalk or marine limestone through the chemical replacement of calcium carbonate by silica-rich fluids. Often contains fossilized remains of siliceous organisms.

Uses & applications

Used historically for stone tools and starting fires (flintlocks). Currently used as a gemstone (Jasper/Agate varieties), in road construction, and for primitive technology demonstrations.

Geological facts

Flint was the most essential resource for prehistoric humans to create sharp-edged tools like arrowheads and axes. It produces sparks when struck against steel due to the hardness exposing fresh iron particles to oxygen.

Field identification & locations

Identifiable by its smooth, waxy surface and distinct conchoidal (shell-like) fractures. Often features a white weathered outer 'rind' or cortex. Commonly found on beaches near chalk cliffs or in gravel deposits.