What Causes Dryness And Pigmentation On Knuckles And Back Of Feet?

well. My skin on my knuckles, back on my feet and belly button are really really thick and overly pigmented. They re a dark brown while my skin is butterscotch colored. I can literally pinch it really hard or remove it: it won t hurt, it won t bleed... it s like I don t have no nerves in these areas. I m really confused especially since I went to the dermatologist once and the medicine he gave didn t even work. I can t find anything on the net either. My skin is not dry in these areas. Help?