Fossiliferous chert from the Pennsylvanian of Ohio, USA.

"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.

The rock shown above is weathered chert from the Upper Mercer Flint, a somewhat persistent horizon of chertified marine fossiliferous limestone in east-central and eastern Ohio called the Upper Mercer Limestone. The Upper Mercer is usually a black flint with whitish speckles (= often body fossils and fossil fragments). This sample is quite fossiliferous. The prominent, gray-colored fossil is a brachiopod shell. Brachiopods are sessile, benthic, filter-feeding lophophorates with two shells. They first appear as fossils in the Cambrian and were abundant on many shallow seafloors during the Paleozoic. They are scarce in modern oceans.

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