Flint Ridge, Ohio is world-famous for having beds of extremely colorful & complexly-patterned chert. “Flint” used to be an academic geology term that, according to one early defintion, referred to dark-colored chert (= cryptocrystalline quartzose sedimentary rock). Nowadays, flint is supposed to be an archaeological term only, referring to worked chert artifacts. British geologists still use “flint” as a geology term, especially when referring to very dark-colored chert nodules in the Cretaceous-aged chalk beds of southern Britain.
For flint knappers and mineral collectors, “flint” is perceived to be high-quality and high-purity, while “chert” is low-quality and low-purity. These are arbitrary, imprecise, not well-demarcated definitions. “Chert” is the proper term for cryptocrystalline quartzose sedimentary rock.
“Flint”does appear to be used (grudgingly) as a valid academic geology term when referring to the chert beds of Flint Ridge, Ohio. Flint Ridge is an ~east-west trending ridge in southeastern Licking County and western Muskingum County in east-central Ohio, USA. Other Pennsylvanian-aged chert horizons in Ohio and surrounding parts of the Appalachian Basin have also been called "flint" in the geologic literature.
The “Flint Ridge Flint” is properly known as the Vanport Flint (lower Allegheny Group, Desmoinesian, upper Middle Pennsylvanian). The Vanport Flint consists of laterally discontinuous meganodules of secondarily chertified limestone. Original limestone textures and clasts are visible in some samples (for example, oolites, oncolites, and fossils).
Vanport Flint - this is the largest flint-collecting pit at Nethers Flint Quarries near the eastern end of Flint Ridge, Ohio (as it looked in mid-April 2008). This site is intensely collected. High-quality, variably colored and patterned flint is abundant here, as are quartz crystal masses from irregularly-shaped geodes. Turgite is encountered here.
Vanport Flint - loose flint pieces at Nethers Flint Quarries, Flint Ridge, Ohio, USA.
Vanport Flint - in-situ flint outcrop in main pit at Nethers Flint Quarries, Flint Ridge, Ohio. The flint color here is off-white. The yellowish-brownish limonite staining is developed along fractures/joints.
Vanport Flint - megaquartz-lined cavity in flint at Nethers Flint Quarries, Flint Ridge, Ohio. Irregularly-shaped geodes are moderately common in this unit. Individual cavity-lining quartz crystals vary in size from nearly microscopic to a couple centimeters in size. Many are clear (“rock crystal”), but some are grayish to blackish (“smoky quartz”), and some are a nice honey-yellow color (approaching “citrine”).
Flint Ridge flint occurs in just about any color of the rainbow. Specimens are sometimes finely-banded (“pinstripe flint”). Flint that's been brecciated and fractured & healed with chalcedonic quartz veins is common. Oolitic flint is somewhat common as well.
Vanport Flint (6.8 cm across)
Vanport Flint (5.9 cm across along the bottom)
Vanport Flint (4.3 cm across along the bottom) - gray pinstripe flint with whitish-gray chalcedony-filled fracture.
Vanport Flint (4.2 cm across along the bottom) - multicolored banded chert with whitish-gray chalcedony- & megaquartz-filled fractures. The brownish, rough-textured areas at the top of the specimen is from extreme weathering. Chert is a very weathering-resistant rock, but it can break down under Earth-surface conditions. Rough-textured, extremely weathered chert is called rottenstone or tripoli. This sample has just begun rottenstone development.
Vanport Flint (5.0 cm across along the bottom) - complex, multicolored specimen of brecciated pinstripe flint with angular clasts cemented by bluish-gray chalcedony.
Vanport Flint (4.6 cm across along the bottom) - the coloring agent of this deep reddish-colored flint specimen is disseminated hematite (iron oxide). “Jasper” has been used to refer to reddish-colored cherts.
Vanport Flint (6.1 cm across along the bottom) - oolitic pinstripe flint with two small reverse faults (see left side of specimen).
Vanport Flint (11.4 cm across at its widest)
Vanport Flint (4.8 cm across along the bottom) with prominent bluish-gray chalcedony-filled fractures.
Vanport Flint (3.5 cm across along the bottom) with pale bluish-gray chalcedony-filled fractures.
Vanport Flint (5.6 cm across along the bottom) - pinstripe flint with a whitish to pale bluish-gray, chalcedony-filled fracture that has offset some of the bands.
Vanport Flint (left: 4.2 cm across at its widest; right: 3.1 cm across along the bottom)
Vanport Flint (left: 1.8 cm across along the bottom; center: 1.7 cm across along the bottom; right: 1.8 cm across along the bottom)
Vanport Flint (4.0 cm across along the bottom) - pinstripe flint with much of the pinstripe banding secondarily bleached out.
Vanport Flint (6.0 cm across along the bottom) - flint with some megaquartz-lined and megaquartz-filled cavities. The dark reddish, dark brownish, and dark bluish-gray are various iron oxides coating a fracture surface (= the surface facing the viewer). These iron oxides include hematite, turgite, and (apparently) goethite.
Vanport Flint (9.4 cm across at its widest)
Vanport Flint (field of view: 2.7 cm across) - oolitic flint. Oolites are sand-sized, concentrically-layered, calcium carbonate (CaCO3) grains that form by back-and-forth wave action in shallow water settings. This fabric indicates that the rock was originally an oolitic limestone and has been secondarily chertified. These oolites are now filled with brownish-colored iron oxides.
Vanport Flint (field of view: 3.0 cm across) - oolitic flint. The oolites here are small-scale weathering centers, resulting in a rough appearance and secondary porosity - the beginning of rottenstone, or tripoli.
Vanport Flint (field of view: 4.5 cm across) - oolitic pinstripe flint. The oolites have been preferentially weathered and eroded, resulting in secondary oolitic porosity in the chert.
Vanport Flint (10.5 mm across) (above & below) - fusulinid foraminifera in flint. Fusulinids are one of several groups of fossils observed in the Vanport Flint at Flint Ridge, Ohio. This is despite the extensive chertification of the original limestone. Fusulinid foraminifera are a group of large, unicellular, non-photosynthetic eucaryotic organisms that made a calcium carbonate (CaCO3) shell in the shape of a rice grain or football. In transverse cross-section view, the fusulinid tests have a subcircular, coiling shape.
Some info. from:
DeLong (1972) - Bedrock geology of the Flint Ridge area, Licking and Muskingum Counties, Ohio. Ohio Geological Survey Report of Investigations 84.