Yes, I think the ceiling should be the same type of rock as used elsewhere.
Instead, why not modify the floor? I assume the character will only be walking there, so it would help display where the character can walk, if you contrast whatever you add to the floor with the rock base.
On another note, the tiles are rendered excellently, but are quite boring, I don't think some experimenting with colors and such would hurt. Although maybe it's just the lack of surrounding elements that add to that boring feel, or if you were going for cold, dark, and gloomy, this isn't too bad, but that's not to say it can't still use a little something.