I stripped a Slate floor about 2 years ago.  Shortly afterwards a few spots showed up in the center of a few stones.  You could wipe them off but they would come back.  I tested the Slate for moisture with a moisture meter, no moisture.  2 years later we went back and cleaned the black Slate and sealed it with an impregnator sealer.  The Slate is inside of the home.  Approximately 2 weeks later more of these dark spots showed up.  No moisture.  The floor is about 40 years old and had a wax coating on it for about 35 years that we removed.  Any ideas?

Thanks