Dive underwater and search for blue shards. Then, once you've found one, go to Hunter's House (to the left of Mossdeep city, on route 124), and he'll trade you a water stone for it.
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Exchange Blue Shard with the Waterstone. You'll find waterstone in Abandoned Ship, too.
For which game? Pokemon Red Pokemon Blue Pokemon Yellow Pokemon Gold Pokemon Silver Pokemon Crystal Pokemon Sapphire Pokemon Ruby Pokemon FireRed Pokemon LeafGreen Pokemon Emerald Pokemon Diamond Pokemon Pearl
Its evolve to Vaporeon from eevee
its not an emerald its RAYQUAZA the main legendary Pokemon in Pokemon emerald.
You can get one in Solaceon Ruins with the dowsing machine app on the poketech.