The Stone Block
as seen in Paper Mario
Stone Blocks are gray stone blocks that appear during the events of Paper Mario. Stone Blocks appear in various locations, blocking Mario's way. They most abundantly appear in Dry Dry Ruins, as these blocks can't be pushed, smashed by the regular Hammer, or be destroyed by other means. The only way to break them is after Mario obtains a Super Hammer from Dry Dry Ruins. Stone Blocks sometimes contain enemies, such as a Stone Chomp in Dry Dry Ruins. This happens rarely, though.
In Paper Mario: The Thousand-Year Door, an Ultra Hammer is required to break Stone Blocks. However, the Ultra Hammer appears late in the game, and is needed to smash only a handful of Stone Blocks.