sedimentary
Flint / Chert
Microcrystalline quartz (SiO2)
Hardness of 7 on the Mohs scale, typically dark grey, brown, or black with a lighter chalky cortex, waxy to vitreous luster, cryptocrystalline structure, conchoidal fracture, specific gravity around 2.60 to 2.65.
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Physical properties
Hardness of 7 on the Mohs scale, typically dark grey, brown, or black with a lighter chalky cortex, waxy to vitreous luster, cryptocrystalline structure, conchoidal fracture, specific gravity around 2.60 to 2.65.
Formation & geological history
Formed through the replacement of calcium carbonate by silica-rich solutions in marine environments (such as chalk or limestone deposits) during the Cretaceous period or other geological eras.
Uses & applications
Historically used for making stone tools, arrowheads, and fire-striking; currently used in aggregate, abrasive grit, and occasionally ornamental lapidary work.
Geological facts
Flint produces sparks when struck against steel due to its high hardness and brittle conchoidal fracture, making it essential for ancient fire-starting and early firearms.
Field identification & locations
Easily identified by its distinctive conchoidal fracture, waxy luster, and often being encased in a white chalky cortex. Common in chalk formations worldwide, particularly famous along the cliffs of Dover, UK.
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