Betonart Audio Arrivato Speakers...Cabinets made of Cast Concrete!..

There have been a few. I own the Celestion Kingstons that are made of a man made synthetic stone that is like concrete, and it does free up the sound from the enclosures. 

I heard the Acora Acoustics granite speakers and was pretty impressed. 

Mark and Daniel! That was it. They make speakers out of what's essentially formica and CAM process granite.

Cast concrete technology has come so far that Wilson and Focal were both looking into it. And, rumor has it, Revel and B&W. But they all dropped the idea upon learning the speaker cabinets would have to be 100 inches thick in order to meet the approval of the Kenjit Society.