In the past in some cultures there has been a policy of 'free' coinage by the government (sometimes with the payment of a small fee) for metals like gold and silver.

This fee is called seigniorage: the revenue or a profit taken from the minting of coins, usually the difference between the value of the bullion used and the face value of the coin.

Coins may depreciate through wear or in the case of gold it has been subject to 'sweating' where a small amount is shaved or removed by intermediate users.

"In the U.S. neither paper currency nor deposits have value as commodities. Intrinsically, a dollar bill is just a piece of paper. Deposits are merely book entries. Coins do have some intrinsic value as metal, but generally far less than their face amount. What, then, makes these instruments 'checks, paper money, and coins' acceptable at face value in payment of all debts and for other monetary uses? Mainly, it is the confidence people have that they will be able to exchange such money for other financial assets and real goods and services whenever they choose to do so. This partly is a matter of law; currency has been designated 'legal tender' by the government - that is, it must be accepted."