Stained corning ware?
You might have scratched off the first layer.
If it's "real" corning ware (aka pyroceram), you might be able to remove the metal marks. I've had really good luck by:
1. first scrubbing with a baking soda paste and a non-abrasive scrubby. I use a sponge made by 3M called O-Cel-O. It's a sponge with little plastic nubs that are very good at removing the most stubborn marks.
2. Buy a ceramic top cleaner like Weiman or Ceramabryte or even Corning ware cleaner (sold in the factory outlets). Use the O-cel-o sponge with the ceramic top cleaner and really scrub. That usually gets off the marks, even black marks on the bottom of French White corning ware. Make sure that you thoroughly clean the corning ware with dish soap and water after you use the ceramic top cleaner. You want to wash it all off.
If these two steps don't work, I'm not sure what else will get it out.