The Stone Mask is one of the 24 Masks in Majora's Mask.
Road to Ikana Canyon
Invisibility to enemies
Obtaining the Stone Mask
Majora's Mask (N64)
The Stone Mask can be acquired at any point after Link learns Epona's Song.
Using the Lens of Truth, Link can see a Clock Town guard at the southern end of the base of the cliff wall in Ikana Canyon. The guard's name is Shiro, and he explains that he has been stuck there for many years, and nobody had stopped to take notice of him. He thinks this is because his appearance is as dull as a stone. He asks Link for a medicine that fills him with energy. If Link gives him Red Potion, Shiro gets his strength back and stands up, thanking Link and handing over the Stone Mask in return.
Majora's Mask 3D
Shiro is located under the tower in the centre of the Pirates' Fortress, but will still give Link the Stone Mask in return for a Red or Blue Potion.
While wearing the Stone Mask, Link will not be attacked by common enemies. In addition, he will go unseen by Gerudo Guards at the Pirates' Fortress in Great Bay Coast, allowing him to walk by without fear of being thrown out. By equipping the Stone Mask, many sets of common enemies throughout the entire game can be skipped making this mask one of the most useful non-transformation masks in the game. The Stone Mask also prevents Leevers from spawning when Link walks on areas that would typically spawn them.
The Stone Mask is not universal, as certain enemies still attack Link; for example, the three Gerudo Pirates in the Pirates' Fortress, who comment on the mask and note that it doesn't fool them.
In Majora's Mask 3D, the Stone Mask attracts the Bashful Angler fish in the Ocean Fishing Hole.