When in Montana the one stone to find is Montana Agate. Apparently once prolific in and along the Yellowstone river now very rare indeed. I cheated I purchased thing slices from a couple different rock shops in Montana. Some of it is very scenic and other more patterns, another of nature's little amazements which is hidden in the disguise of just a old grey rock, unassuming until you break it open and slice it thinly then we see the full magic.
Thendara's Blog
The odds and ends in the creative world of Thendara M Kida-Gee.