I'll be serious and just say that metal is mostly reflective, so what you may actually perceive as proper shading from light are just reflections of the surroundings. So if there was an object in front of my gold bar, the darker side would be completely appropriate. Now I'm just justifying shoddy technique like mine, but just saying
In fact, these reflections are mostly what cause the contrasting effects in metals(from an object being reflected, but being "dark" on the metal against the natural colors of the metal and reflecting light. Pretty much silhouettes, almost.