Instantly teleports you to the portal you are attuned to.
You can bind the Stone to any portal or Ancient June Ruins. Talk to any portal Keeper about attuning the stone. Select "Are you sure you want to bind to this Location?" option.
You can use the Stone to teleport to that destination every 30 minutes.
This item is given to you in the tutorial quest, though its properties are unknown at that point. In a later quest the nature of the Stone is revealed, and it is removed from your inventory and placed at your character screen.