The Thief Stone is a standing stone in The Elder Scrolls V: Skyrim. It is one of the first three Guardian Stones the Dragonborn can choose from.
The Thief Stone improves stealth associated skills 20% faster. The Thief skills are:
† Despite Archery falling under The Warrior skill set, it is counted as a stealth skill in this instance only. It is unknown if this is intentional or a bug.
After completing "Unbound," continue to follow either Ralof or Hadvar and they will lead you to it. It is located southwest of Riverwood, and north-northwest of Helgen close to the river.
The Mage Stone and The Warrior Stone can be found at the same place.