title_body: Question about dirt/rust moka pot
upvoted_answer: It's a combination of scale (mineral deposits left behind by water left in the pot evaporating), oxidized aluminum (caused by exposure of aluminum to oxygen), and coffee deposits (coffee oils and solids left behind by evaporating brew / wet grounds).
  I've seen this with several aluminum pots when they get to be years old. Also steel if you don't keep up with the cleaning. You can remove scale with a coffee machine descaler product (recommend DEZCAL by Urnex), oxidized aluminum with a diluted solution of vinegar, and coffee deposits with a mix of vinegar and baking soda (mixed in the pot with your cleaning tool on standby because they will start reacting immediately).
  Just one bit of advice. Use a rotary tool (like Dremel, if you have one) with a buffing pad. You're going to have one tough time cleaning inside those grooves using a scrubbing pad. A rotary tool with a soft attachment like for polishing or buffing will make the job much easier. Electric toothbrushes work wonderfully too in places they can actually fit/reach.