Black granules forming from skin?

So basically, even though I shower every day and use soap, I get these weird little black granules rolling off when I rub my skin. It seems like dirt and grime, but scrubbing does not get it all off. The only reliable method I have found is to use a knife to vigorously to scrape off layers of the black stuff, and it just returns in a few days anyways. Anyone have similar experiences or any solutions?
Black granules forming from skin?
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