Thunderstone

The Thunderstone is a special elemental stone, introduced in Generation I, which when used on Pikachu makes him/her evolve. It is an elliptical-shaped stone that is green in appearance, with a thunderbolt insignia inside of it, and evolves Pikachu into Raichu. In the anime, the thunderstone was never used on Ash's Pikachu, because he never wanted it to evolve. The thunderstone was first seen in the anime in "The Battling Eevee Brothers".