From the Super Mario Wiki
Hoohoo Blocks are round, purple blocks (who, despite their name, are typically referred to as "stones" in-game) found in Mario & Luigi: Superstar Saga. They can be located at the top of Hoohoo Mountain and are used to make Hammers.
When Fawful blocks Mario and Luigi's way to the bulk of the Beanbean Kingdom (where they can continue their mission to save Princess Peach) by putting a rock in the way, they decide that a Hammer can break the rock. Mario and Luigi meet the Hammerhead Bros., who are able to make hammers, but need Hoohoo Blocks to do so. Mario and Luigi go to the top of Hoohoo Mountain to get Hoohoo Blocks so they can retrieve their hammers.
Little is known about the Hoohoo Blocks, and they are apparently very rare (according to the Hammerhead Bros. and Blablanadon). Their origins are unknown, but several can be found in the caldera of Hoohoo Mountain. Also, Dragohoho is able to spit what would appear to be molten Hoohoo Blocks (including pedestal-like ones known as Hoho Stones) at opponents (and he hatched out of an egg resembling the blocks), which may make him the originator of the Hoohoo Blocks; however, Dragohoho was not native to the mountain and turned out to actually be Prince Peasley hideously mutated by Cackletta and Fawful. It is also unknown if all Hammers of Mario games are made from Hoohoo Blocks or just those of Mario & Luigi: Superstar Saga.