Try your luck at prospecting: hammer open the right rocks and you'll find double-terminated quartz crystals, aka Herkimer diamonds. Two open pits are easy to reach. The first is adjacent to the gift shop. Upstairs you can watch an explanatory video or stroll through multiple scientific displays, including exhibits about dinosaurs and fluorescent minerals. The mines are just south of Middleville.
Rte. 28 between Rtes. 5 and 29, Herkimer, New York, 13350, USA