How do you get eviolite in Pokemon ORAS?

Question

In details please! Not just the route.

Answer 1

There's a house on Route 123, the same house you get Gyaradosite from. Talk to the Fisherman. Pick Magikarp in response to his question and you will get the eviolite.

Source.

Answer 2

The Eviolite is inside the 123 Go Fish shop, but you can only access this by entering from Route 123 (just Surf south of Mt Pyre's entrance and you'll be at Route 123). Then you'll need to work through Route 123 going westward until you find the shop. To do this you'll need to have both Surf and Cut available.

Once you're in the shop, speak to the fisherman NPC and choose "Magikarp" as the response. This will get you the Eviolite.

Here's a step-by-step tutorial if needed: https://guidestrats.com/pokemon-oras-eviolite/