Flint (chert) from the Pennsylvanian of Ohio, USA. (~6.8 cm across at its widest)

"Flint" is the official gemstone of Ohio. Flint is actually chert (the two terms are synonymous, despite what anyone else might say), a cryptocrystalline, quartzose sedimentary rock. High-quality, colorful, multicolored, and multipatterned flint is moderately common at some Ohio localities. A couple famous flint occurrences in east-central Ohio include the Vanport Flint at Flint Ridge and Nellie Blue Flint in Coshocton County.

Nellie Blue Flint is essentially restricted to the Nellie area of Coshocton County. It consists of attractive, frequently complexly-patterned, dark bluish to bluish-black chert. Fractures and cavities (vugs) are often present and have been filled or nearly filled with pale bluish-gray chalcedony and/or megaquartz (= visible hexagonal quartz crystals). Body fossils and trace fossils can be present.

Nellie Blue is a local color variant of the Upper Mercer Flint, which is usually a black flint with whitish speckles (= often body fossils and fossil fragments). The Upper Mercer Flint is a somewhat persistent horizon of chertified marine fossiliferous limestone in east-central and eastern Ohio called the Upper Mercer Limestone.

The orangish-brown coloration on this specimen is iron oxide staining.

Stratigraphy: Upper Mercer Flint (= chertified Upper Mercer Limestone), upper Bedford Cyclothem, upper Pottsville Group, Atokan Series, lower Middle Pennsylvanian