Trade starts with bartering commodities and collectables. Metals start being used in trade. Gold, having superior stock to flow ratio, becomes the premier store

Trade starts with bartering commodities and collectables. Metals start being used in trade. Gold, having superior stock to flow ratio, becomes the premier store of value. People hoard gold when they can, and get rid of the coins of lesser value. This is how you end up with the US government installing a military base over a huge stash of gold, called Fort Knox. They don't do the same for silver and copper.