Thunderstone can be used on Pikachu and Eevee, it evolves Pikachu into a Raichu and Eevee into a Jolteon.
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Pikachu+Thunderstone=Raichu Eevee+Thunderstone=Jolteon These are the only 2 pokemon that evolve with the thunderstone in pokemon emerald
thunderstone cave
Bidoof
catch a pikachu in the safari zone and use a thunderstone on it. catch a pikachu in the safari zone and use a thunderstone on it.
Win the pokeathlon afew time then use the points to buy a thunderstone from the prize corner