If you Google "polishing granite", you'll find all kinds of articles and tools for doing that.
However, I think your best bet would be to take some pictures of your countertops down to any of the companies in your area that make headstones. Most headstones are made from grey, red or black granite, and the companies that are in this business will also have plenty of experience repairing and polishing damaged granite.
If you don't get any other responses in here, the companies listed under "Memorials" in your yellow
pages would be the people I'd first turn to for advice because they won't be in the business of polishing granite counter tops, and therefore won't be more interested in selling you their services than giving you advice on how to do it yourself.
You might also want to give these Canadian guys a call since they have a toll free 1-800 phone number that hopefully works in the USA:
Bomats - Grinding & Polishing Equipment