Stone Block

Stone Blocks are large, grey blocks that appear in the Paper Mario series.

Paper Mario
Stone Blocks first appear in Paper Mario where one is first seen near Jr. Troopa's Playground. However, Mario's regular hammer is unable to break it forcing him to leave it as it is. However, Mario can then start to destroy Stone Blocks once he gets the Super Hammer in Dry Dry Ruins during Chapter 2.

Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, Stone Blocks appear much less often and Mario now needs the Ultra Hammer to break them. The Ultra Hammer is found in Rogueport in the large treasure chest in the main area. However, Mario will need the Ultra Boots from Riverside Station during Chapter 6 to get to the Ultra Hammer and start breaking Stone Blocks.