The purple Stone ball was an item used in the Death Plateau quest. The five stone balls had to be arranged in the correct order on a mechanism inside Burthorpe Castle in order to unlock the door to the equipment room.
The combination to the positions of each stone ball was won from Harold in the Toad and Chicken pub by playing a dice rolling game. When Harold lost sufficiently he declared that he had no money and gave the player an IOU instead. Inspecting the back of the IOU revealed that it was really the combination to the lock.