If your marble top is polished and you use a filler to fill the crack, you will need to sand down the excess. A razor will scratch marble. If it is sanded down then you need to repolish. This is a tricky area, because it is hard to match or blend the surrounding areas. The next situation would be to resurface the whole countertop. I suggest calling a qualified local restoration specialist in your area to fix if you are hesitant.