Agate is classified as semiprecious stone, and Nebraska has an abundance of it, especially in the Oglala National Grasslands north of Crawford.
The stone is described as a variety of silica composed of alternating layers of variously colored chert of granulated crytocrystalline quartz. "Agate" is the term generally applied to chalcedony having a definite color, pattern, layers, bands or markings of some description. An agate may be formed by silica-bearing waters that are deposited in and slowly fill cavities in the earth.
Agate is a variegated quartz noted for its layered varieties. In most specimens, the bands are very coarse and differ in color and translucency, as well as in compactness and porosity. The prairie agate, distinguished from most other agates because it seldom has these bands, is still very colorful, has a rounded irregular shape and is popular for jewelry.