care of marble and granite

Our marble around the sink seems dull, when I wet the area it is fine, What needs to be done.
And my granite countertops, how often to seal, can I do this?
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I purchased a granite cleaner/polisher from the place where my granite was purchased. I used it once a week when it was new, and it seemed to leave a kind of sandy feeling when rubbed dry. I was told that I may have received a bad bottle of the cleaner. After speaking to several people, they said to only use vinegar and water to clean the granite. Lately, I've been wondering if this is good advice. I'll look forward to responses on this subject.
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