This is a large polished half of a DRYHEAD AGATE from the Pryor Mtns of Montana, measuring 5.25 x 5.0 inches across the face in largest dimensions. It was in the Roger Pabian Collection (see below) and published in his booklet "Banded Agates, Origins and Inclusions" (1994) at page 27. It is also featured in the online Agate Lexicon collection photos at:
and searching for, United States, and then filtering Montana.
the photo is also linked here:
I acquired this piece recently for my Dryhead Agate collection, but am trying to downsize the larger pieces. THIS IS A GREAT SPECIMEN FOR ANY SERIOUS COLLECTOR WITH IMPORTANT PROVENANCE AND PUBLISHED.

About Roger Pabian:
Roger Pabian, who originally compiled these (Agate) collections, passed away in December 2010. He was formerly a emeritus professor in the School of Natural Resources and Conservation and Survey Division. Pabian was active until the end of his life in sharing his passion for agates, fossils, gems and other geologic finds. He recommended learning about naturally occurring gems and where to find them as an excellent family activity, and Charles Wooldridge, president of the Lincoln Gem and Mineral Club, noted that he made many friends over several decades by sharing his knowledge and enthusiasm. Pabian was a charter member of the club, served as president four times, and helped organize and present programs.

Pabian wrote general, technical and peer-reviewed publications throughout his career. Shortly after he retired, the British Museum of Natural History contacted him about co-authoring Agates: Treasures of the Earth, which was published in 2006.

He regularly led tours for groups such as the Nebraska Academy of Sciences, the Geological Society of America, the Nebraska Geological Society, and UNL's Geology Department (now the Department of Earth and Atmospheric Sciences).
The above cited from the University of Nebraska Lincoln Pabian acknowledgments page at:

DRYHEAD AGATE Published Pabian Collection