Prairie agates from the plains of western South Dakota. The biggest one I hauled home from a hike less than a mile from the house. That piece of land has recently been fenced for summer cattle pasture, so it is no longer accessible for rockhounding.
Agate is a variegated quartz noted for its layered varieties which can differ greatly in color and translucency, as well as in compactness and porosity. The prairie agate has a rounded irregular shape and is popular for jewelry, being considered a semiprecious stone. This one is a nice big specimen about the size of a football, I was lucky to stumble upon it.