Well, that's an interesting thing to find! You know already that the ring can't be solid gold because gold doesn't rust like that. Gold may develop a little patina, but that's about it. I would definitely clean the ring, take off the rust and see what's there. It doesn't have much value as a piece of rusty jewelry. Only if a ring has a beautiful patina or nice wear marks that give a lot of character from age should it not be polished. But I would clean it up and see what story your dug up treasure tells! It can only add value to a piece of rusty metal. Robert