Once you find a pokemon of the fire type which has something to evolve into, (like a numel or torchic or something) you then go into the items pocket in your bag, select the fire stone, and click use. It will then go to the screen with your party of pokemon. Select the fire pokemon mentioned earlier, and it'll evolve.
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Use a fire stone on Vulpix.
you can use a water stone on lombre
No, neither of them evolve from the use of a Fire Stone, though you can have them hold one. Torkoal doesn't evolve at all, and Slugma evolves at level 38.
get eeve and use a fire stone
Dawn stones aren't available in Pokemon Emerald. Use Mystery Gift.