Nothing. An everstone is just a hold item to stop the Pokemon from evolving.
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An everstone is used to prevent evolution
Lombre is one of the pokemon that do not evolve by level he evolves by using a water stone.
There are two in granite cave one by going up the steps on the first floor and another by using a mach bike to fully explore the cave.
The Pokemon that evolve using a water stone: Eevee Starmie Lombre Poliwhirl, too...
Seadra can be evolved into Kingdra by using a water stone