How do you get Everstone on Pokemon Blue?

In Pokemon Blue, the Everstone can be obtained by defeating wild Geodudes and Gravelers in Rock Tunnel. These Pokemon have a 5% chance of holding an Everstone when encountered. The Everstone prevents a Pokemon from evolving when held, making it useful for trainers who want to keep a particular form or moveset. Remember to save your game before battling these Pokemon to increase your chances of obtaining an Everstone.