If you want to get the Gem of Persuasion you have to go at first to Crystal Caverns and kill the Rygorr Orks until a Bottle of Karsinic ( no drop ) drops. Any kind of the Orcs can drop this.
Now you head to Thurgadin and give this bottle to Erdarf Restil, who is in the bar called "The Holy Harbinger", after it you have the gem.
Any idea who/what drops this and where? Could it possibly be a foraged item...? I know I have foraged a ton of Small Chunk of Velium ( https://everquest.allakhazam.com/db/item.html?item=3840 ) in Crystal Caverns so I would not be too surprised if it was foraged.