“Large alluvial cobble of “Prairie Agate” (two sides), most likely from the Ogallala National Grasslands in northwest Nebraska, in the same general area that Fairburn Agates (or “Fortification Agates”) are found (and also in the Black Hills in South Dakota). Some are also found occasionally in the Platte River alluvium all the way to the eastern part of the state. Prairie Agate was designated the “state rock” of Nebraska in 1967. That fact is a hint that it is not really agate (a mineral), but chert (a rock). It is very hard and takes a good polish, but the layers are generally not as distinct as banded agates usually are. Prairie Agate is believed to have originated in mostly marine sedimentary limestones of Middle to Late Pennsylvanian and early Permian ages that were deposited in the Front Range and Hartville Uplift of Wyoming and in the Black Hills of South Dakota. These rocks were subsequently eroded and the hard siliceous materials were carried into Wyoming, South Dakota, and Nebraska by streams that deposited basal conglomerates in the Chadron Formation of Oligocene age. (Source: Nebr. Geol. Survey “Agate Lexicon”). ”