Flint from the the Pennsylvanian of Ohio, USA. (~13.5 centimeters across at its widest)

Flint is the "official" state gemstone of Ohio (actually, there's no such thing as "official" anything). "Flint" is sometimes used as a lithologic term by modern geologists, but it is a synonym for chert. Flint and chert are the same - they are cryptocrystalline, quartzose sedimentary rocks. Rockhounds often assert that flint is high-quality while chert is low-quality. Some geologists assert that "flint" implies a biogenic origin and "chert" implies a chemical origin.

Many cherts do have a chemical origin - chert nodules are moderately common in some limestone units. The nodules form during diagenesis - pre-existing silica components in the carbonate sediments are dissolved, mobilized, and reprecipitated as chert masses. Some cherts do have a biogenic origin - for example, radiolarian cherts (rich in radiolarian microfossils) or spicular cherts (rich in siliceous sponge spicules).

The most famous flint deposit in Ohio is Flint Ridge, in Licking County. At this locality, the Middle Pennsylvanian-aged Vanport Flint is exposed in several places. The geologic literature on the Vanport Flint is relatively sparse, with inaccurate, incomplete descriptions and characterizations. For example, the literature describes the Vanport as a sheet of flint at Flint Ridge - it's actually a meganodule horizon. Other descriptions refer to the chert as the remains of siliceous sponges. In reality, siliceous sponge spicules are quite scarce in Vanport samples.

Two graduate student projects during the 2000s, conducted at two different universities, had very different conclusions & interpretations about the origin of the Vanport Flint. A 2003 study concluded that chert at Flint Ridge is biogenic in origin. A 2006 study concluded that the chert is chemical in origin.

Studies done by geologists at Ohio State University at Newark indicate that the Vanport Flint has a relatively complex history, the details of which are still being worked out.

Modern flint knappers value the Vanport Flint for being multicolored and high-quality (= very few impurities). With artificial heating, the flint is more easily knapped into arrowheads, spear points, and other objects. Prehistoric American Indians quarried the Vanport Flint at many specific sites on Flint Ridge. Old Indian flint pits can be examined along hiking trails in Flint Ridge State Park ("State Memorial"). Many authentic Indian artifacts found in Ohio (arrowheads & spearpoints - "projectile points") are composed of Vanport Flint.

Much of this sample has numerous thin, closely-spaced lines and is known as "pinstripe flint" or "Nethers flint". The striping resembles Liesegang banding in sandstones, which results from groundwater moving through porous rocks and precipitating minerals - usually iron oxide (e.g., see: <a href="https://www.flickr.com/photos/jsjgeology/albums/72157647217431341">www.flickr.com/photos/jsjgeology/albums/72157647217431341</a> ). Flint/chert, however, has low porosity and very low permeability, so pinstripe banding must have a different origin (?).

The reddish and yellowish-brown colors are from iron oxide impurities (hematite and possibly limonite).

The light-colored, vermiform structure at left is a fracture filled with chalcedony (= translucent, microcrystalline, fibrous quartz) and megaquartz (= macroscopic quartz crystals in chert).

The whitish-colored area at right consists of brecciated flint, with the angular clasts well cemented together by chalcedony and minor amounts of megaquartz.

Stratigraphy: Vanport Flint, Allegheny Group, upper Middle Pennsylvanian