The Moka pot is made up of aluminum; the opaque finish is probably aluminum oxide or sapphire. This is why you want that in all your aluminum cookware because that is what stops the aluminum from percolating out into your food. It's the reason why people always advise you to boil water in a new aluminum pot to get this to form. Don't use vinegar or lime-away to remove this beneficial layer. But still, if you want to remove it, google "season my Bialetti," and you will get that residue.