The Diamond Of Deceit is a magic gem that appears in the episode "Smurf On The Run" which is used by the Spirit Of The Scales to maintain the balance between truth and lies along with the Topaz Of Truth. As long as the diamond is intact, if the topaz was destroyed, everybody in the world would be subjected to telling lies. There's no mention of what would happen if the diamond was destroyed, though given what was shown in the episode, one would pretty much guess that everyone in the world would be forced to tell the truth.