My wife thinks I'm weird because I have a rock collection. Ok... maybe it is a little nerdy. But I really enjoy looking for Montana moss agates on the river.  I get the same kind of thrill when I find a big agate that I get when I hit a keno machine for 5 out of 6 numbers. I'm always dragging home pockets full of agates and petrified wood when we come back from a day on the Yellowstone.  My five gallon bucket is overflowing, to the point of needing another bucket.

Unpolished, they're not really worth anything. Sliced and polished they're worth maybe a couple of bucks. They're not exactly rare, but it's the fun of finding them that I enjoy. Montana is a great state for rock hounding in general. Besides moss agates, sapphires can be found in the gravel beds of Lewis and Clark County, near Phillipsburg, and other parts of the state. If you don't wanna spend too much time looking, you can even buy bags of dirt in Phillipsburg and are almost guaranteed to find some. The kids love it. Calcite, opalite and garnets can be found in the vicinity of Ruby River Reservoir south of Alder. Smokey quartz and amethyst can be found in the hills east of Butte.  There are a number of books out there for rock hounding in Montana, if you're interested. I have this one.

If dino's are more your speed, you have a decent chance of finding dinosaur fossils if you search the hills and washes around southeast Montana and up around the Fort Peck area. Rock on.