Where to find Stone in the Old Camp
Stone is at the Old Camp, in the inner ring. He is a blacksmith and works on the armors for the guards.
You can meet him only after passing the entrance guarded by Thorus, so you'll have to join a guild or bring 1,000 ore pieces to Thorus to buy him out.
Pass the gate, turn left and you will find a wooden roof on the left, where you will see Stone working on the forge.
Where to find Stone in Chapter 6
Since chapter six, Stone will be inside the prison of the Old Camp. To find him, teleport to the Old Camp. The teleport rune is inside the old tower of Xardas.
After teleporting you will find yourself in the building of the Fire Mages. Go outside, walk straight forward and pass the door in front you of you, that leads to the guards dormrooms. Reach the end of the hallway, turn right to go down towards the prison, kill the guards and open the last door on the left to free Stone. To opem the door, you must have collected the key by killing Gomez.