The new Diamant pads that will be available in May '13 work well. I used P500-P4000 on Onyx on a recent project and it works well and fast. I got the pads from a mole in the UK (thanks, mole!) You probably could do the same with Platin pads which are currently available. Diamant pads have industrial diamonds in the pad. Keep the surface wet (not splashy wet) and it works very quickly. For what you are doing, the higher grits might be best to simply bring back the polish. P500 could have you dishing if the marble is very soft.