you didnt say what type of metal it is. Also.....is it jewelry? or is it a real arrowhead? Be careful what you post, the feds or state could be visiting you if they think you have an indian artifact. I dont know what state your in, but here....we cant dig or keep any. And the laws here are sketchy on those found on private property, but state and federal land is a major NO KEEP, NO DIG. I would advise you find out the laws in your area.
Vaseline is a great way to preserve many metals especially copper. Make sure its been cleaned of any dirt by soaking in hot peroxide, and use a very soft toothbrush with dish soap on it. I do that cycle a few times on very cruddy copper. Dont try cleaning with anything your not sure of, copper is very soft, and damages easily. Hot peroxide and soft toothbrush wont remove the patina, and thats what you want to preserve.