Quick update:  Just trying to follow up various possibilities I went back into Blender.  This time I uniformly scaled the model down 50% using the usual scale tool, then exported with no additional scaling done during export just to see if that perhaps made any difference.  As I suspected, it didn't... same issue still exists and still only with that one Marble MAT.

Could it just be something with my video card, bug in Windows, etc?   Not sure why that would affect just one specific MAT but... I'm running out of ideas.