You get Rock Smash in Mauville City. Talk to the guy in the house to the right of the Pokemart.
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Get the HM Rock Smash
go to the granite cave and have a pokemon with the move rock smash then go to a stone you can smash with rock smash smash it and nosepass may appear
You can break rocks by using the move Rock Smash.
If it is a round boulder then use strength, otherwise if it is a rock use rock smash.
you need some HMS such as dive strenght and rock smash