Lea Blacksack
Well, I'd advise you to begin by questioning the businessmen who used those cheap stones. Try to get the name of their suppliers or at least where the trades took place. They'll probably be reluctant to talk to you, but I have every confidence in your powers of persuasion... And please, try not to get them too angry. We like having them as customers.
You'll find the builders at the North Star here in Maliken, at the Mocassin Inn north-east of the city, and at Madame Isolde's Lodge, in Bolgatur fortress.
Good luck!