"Impervium" is a magical indestructible metal. It always appears as a dark black metal surrounded by yellow/gold orbiting streaks of magical energy.
To see that description in the art, take a look at these concept sketches and zoomed-in finished art for "impervium" cards.
Darksteel Brute is a statue made of darksteel that can come to life to smash some face. Note how well the artist, Nottsuo, captured the glossy black metal.
What's better than an Iron Giant? One made of darksteel! Darksteel Colossus clearly shows the "orbiting streaks of magical energy" mentioned in the style guide.