From the Super Mario Wiki
The Rock Mushroom is an item that has so far only appeared Super Mario Galaxy 2. It turns Mario into Rock Mario. When collected, Mario's hat and overalls turn to stone, and it gives him the ability to encase himself in a Boulder by shaking the Wii Remote. He can also accelerate by jumping and then shaking the Wii remote. In this form, Mario can roll around and jump, damaging enemies and large purple crystals with items inside them. If Mario hits certain crystals or enemies, he will revert back to his normal form, but will still have the rock mushroom. Players can always return to his boulder form by shaking the Wii Remote again. It is possible to turn back to normal by jumping and pressing . This is useful to stop Mario from flying off an edge; however, this move has been disabled during the second mission of Melty Monster Galaxy (A Stroll Down Rolling Lane) - it would make the mission far too easy. The Rock Mushroom plays a major role in the Boulder Bowl Galaxy, and also appears in the Melty Monster Galaxy.
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