Sun Stone

The Sun Stone is a stone shaped like the sun. It appears in the game Paper Mario: The Thousand-Year Door. Along with the Moon Stone, Mario needed the Sun Stone to gain access to Hooktail Castle. Both the Sun Stone and the Moon Stone are located in Shhwonk Fortress and are protected by Fuzzies.

The in-game item description says, "A mystical stone shaped like the sun."

Trivia

 * This item is similar to the Sun Stone from Pokémon Gold and Silver, which is an evolution item which let some Pokémon evolve.