These one inch sweetwater agates are a variety of chalcedony found along the Sweetwater River in central Wyoming. They are translucent, dull gray and are filled with manganese dendrites. They almost always fluoresce.
Pioneers often picked up a few as they traveled west along the Oregon trail. A few even made it to Oregon and California.
A dip slope surface of about 100 square miles was covered with these agates. As a boy, I easily found lots of them while hunting with my dad. In the 70's the whole area became a gigantic summer campground for rockhounds hunting for jade, petrified wood and agates. Today, it's a rare surface find. My sons and I hunted for several hours and only found half of an agate. It's the one in the top left of the photo. It was a discard from the past when a rock hound hammered it in half and didn't like what he saw. Where did all those agates go? And how did they disappear so fast?