sedimentary

Flint / Chert with Cortex

Microcrystalline Quartz (SiO2)

Hardness of 7 on the Mohs scale, waxy to vitreous luster, conchoidal fracture, colors range from dark grey/blue-black to yellowish-brown weathered outer rind (cortex), specific gravity around 2.60 to 2.65.

Identified August 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the chemical replacement of calcium carbonate in limestone or chalk by silica-rich solutions in shallow marine environments over millions of years during the Cretaceous or older periods.

Uses & applications

Historically used by early humans for stone tools, arrowheads, and fire-starting (when struck with steel). Today used mainly for decorative landscaping, gravel, and occasionally lapidary work.

Geological facts

Flint and chert are essentially the same mineral (microcrystalline quartz), with 'flint' traditionally referring to dark varieties found in chalk deposits. It was one of the most critical materials for human technological development in the Stone Age.

Field identification & locations

Easily identified by its smooth, water-worn or weathered chalky outer cortex, sharp conchoidal fracture edges, and high hardness capable of scratching glass and producing sparks with steel. Found globally in chalk and limestone formations and river gravel beds.

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