You don't need a stone you need to trade it to someone then trade it back to you and it will evolve!
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No. the only way to evolve Haunter is to trade it.
Haunter doesn't evolve at any level to evolve your haunter you need to do a link trade with a friend and he he will then evolve
You evolve it into Haunter at Level 25. To evolve Haunter into a Gengar, you need to trade it.
You have to trade to another game to make it evolve.
You will need to level up Haunter to level 50 to evolve it in Pokemon Resolute.