If the surface is all stone, and the oily substance is organic, you probably can get it removed with solvent (isopropyl Alcohol, Acetone or even Goo Gone.
Probably should test the least important areas first to make sure it works without creating more damage...
59 posted on 09/09/2007 7:39:26 PM PDT by John123
("What good fortune for the governments that the people do not think" -- Adolf Hitler)