If your talking about the tan-ish lumpy ones, then as soon as you walk into mt. Coronet a mountain man will give you the hm rock smash. Then once you beat the oreburgh gym you can teach it to a Pokemon. Then put that Pokemon in your party and walk up to one of the boulders. Press a and it will ask you if you would like to smash it. Say yes and your Pokemon will smash it and you can walk by.
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In the game Pokemon Pearl, the HM move Rock Smash is required in order to break the boulders in Mt Coronet. To fully explore the mountain, a players needs Pokemon that know Rock Smash, Strength, Rock Climb, Defog, Surf, and Waterfall.
you can use the hm strength you can smash rocks with rock smash
you must get the hm rock smash, if the bolders are a redish-brown with cracks then you can smash the rocks
You must learn rock smash. You get that from a man standing in the right of you you when you have beaten sudowoodo
If the boulders are smooth then you need to get one of your Pokemon to learn strength to move it and if the boulder is rough then you need to get your Pokemon to learn rock smash ( both of these moves require HMs to be learnt)