Yeah, and if I were a jeweler, I'd recognize diamonds...doesn't mean they aren't worth a lot or that they're common! Remember what was said in the book? That one suit of mail was worth more than everything else in the Shire put together!
I would just simply question the quality of Mithril offered by a Gondolier...
Dwarves...you TRY to be gracious and apologize to them and even offer them gifts, but they can't help acting like...well...DWARVES! ;-)
Oh sure, "Mithril" from a Gondolier is like a "Rolex" from a NYC street corner...