sedimentary

Chert (or Flint)

Microcrystalline Quartz (SiO2)

Hardness of 7 on the Mohs scale, colors range from grey, brown, to dark green or black, waxy to vitreous luster, cryptocrystalline structure, conchoidal fracture with very sharp edges, specific gravity of 2.60 to 2.65.

Identified July 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the replacement of calcium carbonate by silica-rich solutions in sedimentary environments like chalk or limestone, or by the accumulation of siliceous oozes from microscopic marine organisms like diatoms and radiolarians over millions of years.

Uses & applications

Historically used for making stone tools, arrowheads, and striking fire due to its conchoidal fracture and spark-producing properties when hit against steel. Also used in abrasives and occasionally in decorative items or lapidary work.

Geological facts

Chert and flint are chemically identical forms of microcrystalline quartz, with 'flint' typically reserved for the darker varieties found in chalk formations. Prehistoric humans heavily favored chert for stone-age weaponry due to how predictably it fractures into razor-sharp flakes.

Field identification & locations

Can be identified in the field by its distinct conchoidal (shell-like) fracture patterns, waxy luster, and high hardness which easily scratches glass. Commonly found nodules within sedimentary rock formations, especially limestone and chalk beds worldwide.