One formation that has always fascinated me is Devil's Tower in Wyoming. If you are old enough, you might remember it as Mashed Potato Mountain from the movie Close Encounters of the Third Kind.
It was a volcano at one time. As
@KristaFairchild describes, the inner tube of lava cooled very slowly, forming very hard granite. Over time, the softer rock of the mountain eroded away, leaving only that granite tower.
In South Dakota, they found huge crystals of spodumene. One "log" found at the Etta mine is described as 42 feet long. The Black Hills School of Mines in Rapid City has a Geology Museum, as
@Stottie Girl mentioned. It took me four hours to go through it, and I still didn't get to see it all.