AT FIRST The thing had the "burn victim" look, it was not til later that he had the rock look... ie his skin was forming--atleast that is how i remember it..(damn making me look it up)
http://www.chivian.com/chivian/ff/FantasticFour1.html http://www.chivian.com/chivian/ff/FantasticFour2.html
his skin is not "rocky" like we know it
it is not til between 10 and 18 that his skin" hardens" into the rock style that we all know.
issue 12
http://www.chivian.com/chivian/ff/FantasticFour12.html
issue 20- the style we all know
http://www.chivian.com/chivian/ff/FantasticFour20.html
i picked 20 instead of 18 because you se more of him.. but 18 is close as well