To open the Cave of Origin in Pokémon Emerald, you must first go underwater to the Seafloor Cavern. There you must defeat Archie.
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There is no jirachi's cave.
This is because you have to go in the cave at the top of the town then the gym will open.
I don't think you can, in emerald you need the nationaldex and POW! it's open but in ruby and sapphire you can't.
You read the middle braille, head for the middle of the room, and use fly. the door should open, and you'll be able to fight a level 40 Registeel.
you need azelf,uxie,mesprit to open the cave after beating the Pokemon league