crud in the tank, looks like the marble came out this way, notice how the crud follows the glassflow, a cull. i have loads of dug marbles that look just like it, not metal, a chemical reaction. experimental were 99% dug. i have one from the Akro French drain dig that is absolutely wet mint yet it was discarded as a cull like so many were. I'll go ahead and post it and the article from the French drain. they get looking like yours after a long time in a fish tank too.