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Flint is a sedimentary rock consisting of microscopic, nearly undetectable (cryptocrystalline) crystals of the mineral quartz (SiO2). Related to other varieties of quartz rocks, including chert and jasper, flint is a term applied to purer examples of cryptocrystalline quartz rock that exhibit a waxy luster, and to certain uses of the rocks, such as for stone tools.

In Ohio, flint occurs as lenses, nodules, or sheet-like deposits within marine limestones. The limestones were deposited in shallow inland seas that covered much of Ohio during the Paleozoic Era. Sources of the silica (SiO2) for Ohio flint deposits are not well understood. Various sources have been proposed, including both inorganic sources and biologic sources such as spicules from siliceous sponges and siliceous tests (outer skeletons) of phytoplankton. Flint formation likely occurred after the rock was deposited and as part of the process of physical and chemical changes the rock underwent as a result of burial.

Flint is hard (it will scratch steel), but it is also brittle and breaks with a glasslike (conchoidal) fracture that can produce sharp edges. When gradually heated and cooled, flint becomes more homogenous in texture and is easier to knap (shape by chipping). Natural colors resulting primarily from traces of iron minerals are also intensified.

Ohio flint was highly valued by Native Americans, who quarried it from multiple locations within the Upper Carboniferous (Pennsylvanian) rocks exposed in the eastern part of the state. They came in great numbers to Flint Ridge (Licking County) to quarry the colorful flint of the Vanport Limestone. Native Americans used Ohio flint to make projectile points, such as arrow and spear heads, as well as drills and other tools. Early European settlers used the flint as buhrstones (hard millstones) to grind grain. Today, uses of flint are primarily ornamental, such as in jewelry. In 1965, flint became Ohio’s official state gemstone. See also "Quartz."

Composition: Silicon dioxide (SiO2).

Additional Information

Bulletin 69: Minerals of Ohio - To order, contact the Geologic Records Center.

Educational Leaflet 6: Flint: Ohio's Official Gemstone