Yeah, if it wasn't in the lot of MKs with similar colors and av (and I think all of them were dug), I might have had a hard time figuring it out, but still I can see MK in it. This one looks dug too.
I'm guessing it's some kind of soot from burn-off of contaminates or something like that in the tank. I melted a marble with an oxy-acetylene torch once and it changed the color and gave it that metallic look too. Impurities I guess.