In many modern appearances, the X-Man Colossus is made of organic osmium, a particularly dense material. Of course, many of these "scientific" facts about superheroes were conceived long after the fact, like the Hulk's nanotube skin. Further, it's possible that the writers were originally going for a steel or iron look, to fit the Soviet theme (Stalin, meaning steel; the Iron Curtain; and other examples of steel being a Soviet motif).
In what issue was it first mentioned that Colossus was made of osmium? Was he ever described as being a different material?