Step 1 - weigh 6 + 6 on both the sides. Pick the 6 stones on the lighter side.
Step 2 - Then weight 3+3 on both the Sides ( picked from Step 1) . pick the 3 stones on the lighter side.
Step 3 - Then weigh any two randomly from the stones picked from Step 2. If they weigh equal the other stone will be lighter. If not the Scale will itself give you an answer.