From the Super Mario Wiki
“Hoohoo Blocks are hard stones. Hammers made out of those things can break anything!”
—Hammerhead Bros., Mario & Luigi: Superstar Saga
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 are located at the top of Hoohoo Mountain and are used to make Hammers.
After Fawful blocks Mario and Luigi's path by putting a rock in the way, they meet the Hammerhead Bros., who are able to make hammers, but need Hoohoo Blocks to do so. Since hammers seem to be the only way to destroy the block, Mario and Luigi go to the top of Hoohoo Mountain to get Hoohoo Blocks. Little is known about the Hoohoo Blocks, and they are very rare. Their origins are unknown, but several can be found in the crater 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.
Names in other languages
Portmanteau of rigoler (to laugh) and roche (a rock)