To remove rust from marble surfaces use lemon juice and baking soda. One lemon one tablespoon of baking soda mixed as a paste. It works better if you do it in the sunlight. When the mark has been removed (you may need to apply the paste as couple of times) use cream (you know the stuff you get from cows) to polish!!
Neville Beechey PO Box 275 Colac 3250 Victoria Australia