If the relic is over 100 years old, you will likely have to turn it in. Some areas will require you to notify park staff or the authorities if you find something you suspect may be culturally or historically important. Some other areas require you to turn in items that have identifying information on them so they can attempt to locate the rightful owner.  You can take the item to a university or historical society in attempt to determine what it is and how old it might be. Remember, any area on state or federal property where an artifact over 100 years old is found, requires you to turn in the item. If you are searching on private property, so long as you have written permission from the landowner and have worked out a deal with them pertaining to items you find, you do not have to follow the laws governing relics over 100 years old.