Yet, gold was corruptible. It doesn't rust, yes, but as gold coins and nuggets were used, in said use, they would erode slowly. That is one of the main reasons we switched to banknotes. Banknotes originally were basically a bond, a proof from the bank that such amount of gold is held by the bank in your name. People then figured out it's a hassle, so they started exchanging said notes, using it as essentiay cash, money as we know it today. Back in some year, can't remember, money notes stopped being backed in gold, and just became what you said, a number that we give value to, that has absolutely no real hard value behind it but the guarantees of various institutions (Central Banks of the world) and inherent market laws. That's why value of money can tank overnight due to politics, economy upset etc.