But you can also do this:
Keep the real stuff for the Mace of Disruption +2.
Get the fake stuff from the ore merchant and show it to Sarles.
He'll notice that it's fake and he'll leave. But that's ok.
Now go to the temple that hired you to do this in the first place and
tell them what happened. You'll show the fake stuff to the cleric your
talking to and then a high priest joins the conversation.
He thinks the lump of fake ore is a fine work of art
and wants to keep it.
So, in the end you get the mace and a bunch of exp for helping the temple...