Gresham's law is a narrow, stubborn observation. When two moneys are forced to pass at the same face value, and one is better to hold, people spend the worse one and keep the better one. The shop sees the melting coin. The good coin leaves circulation and sits in a drawer. Law did not make the good coin worse. It made spending the bad one the rational move.
Worth knowing
If both must spend the same, people hand over the worse money and hoard the better.