Customs and Border Protection also states that if someone introduced a “foreign currency for spending” made of gold, for example, using gold coins as a country's legal tender, that gold would be considered a form of currency and would be treated the same as cash. The Office of Customs and Border Protection suggests that if a person is not sure if their gold is considered currency, the safest thing to do is simply to declare gold to avoid making a false statement. The gold reserve held by the Treasury Department is partially offset by the obligation for gold certificates issued to Federal Reserve banks at the legal rate, which the Treasury can exchange at any time. Some people may think that gold gets the same treatment as money when it goes through customs, but slightly different rules apply when bringing items made of gold to the United States.
Customs and Border Protection: Gold that isn't considered a form of currency doesn't require a tariff tax, but the agency recommends declaring gold anyway.