If not sealed, and the stone becomes stained, you are faced with either replacing the stone or an expensive time consuming effort to remove the stain with no guarantee of success.  Stain removal is one of the only services that most stone craftsman will NOT guarantee.  In order to remove a stain, one must identify and prepare a chemical cocktail designed to dissolve the invading spill material.  This chemical cocktail is applied via a poultice.  The poultice must be left in place overnight.  If the cocktail prepared was appropriately selected, it will begin to draw out the stain.  This process may have to be repeated one to three times and in some cases as many as 10 times before the stain is completely removed.  While the poultice is down this surface cannot be used, as such one may lose the use of a counter or floor area for several days.  Further in many cases, the chemical cocktail includes aggressive solvents or odorous chemicals.  Some chemicals may even damage the stone; requiring restoration after the stain is removed to return the stone’s original finish.