ThunderStonein Pokémon Polished Crystal

Evolves certain kinds of Pokémon. Works on Pikachu (Raichu), Eevee (Jolteon), and Magneton (Magnezone). Bill's grandfather on Route 25 gives you one for showing him a Pichu! If you're evolving Pikachu, wait until it has all the moves you want. Raichu doesn't learn anything new by level up.