I see what you're seeing, it doesn't appear to be an average stain it looks like it has raised edges or is an imprint you can see light reflecting off the edge. I don't know what it is but just want to let the poster know I see that it could look like the metals didn't properly mix like a can of paint that wasn't shaken enough. Lol I know it can be frustrating when nobody seems to see. That being said I have no clue.