Sorry, no photos but I looked at a job I did a few months ago (flatten to 800ao hone) with a very dense marble from Greece that is (for lack of a better phrase) “oxidized”. All areas that are not under roof are turning light, as it if it were etched. When it is rubbed with a finger there is a chalk residue. This residue washes off but to fully remove it takes some abrasive action, I did a small sample with a 800 grit Spinflex pad. I have seen this problem a lot with limestone and especially travertine. I think it is just a natural erosion, like a “micro-spalling”   this is not from the pool water, but from rain water, it is not so smoggy here, so I don’t know if it would be acid-rain, but it could be. Any insight to this problem?  Any solutions?? They don’t want it to return.