In Mallrats, Brodie asks Stan The Man if the Thing's dork was also made outta rock, to which we never got an answer. Until now.
"I never gave it a thought. I guess common sense would say it was made of orange rock too." said Lee. "But I always thought it was more interesting to think about Reed Richards. As you know, he had the ability to stretch, and sexually, that would seem to be a great asset in many areas."
So there ya go. The answer we all knew already, but said by The Man himself.