I am just wondering where it says that the Federal Government cannot make something other than gold and silver legal tender. I know the Constitution says the states can't make something other than gold and silver legal tender, but what about the federal government? It's not a state's right obviously because the Constitution gives the Congress the right to coin money, so where is the idea that the Federal Government can only make gold and silver legal tender?
The power has to be enumerated in the Constitution. Legal tender laws fall under property rights which are a State issue like land, contract and tort law. Why? Because they are
remedies for contract and tort suits. However, States are restricted by the Federal Constitution in what they can make Legal Tender (Art 1 Sec 10).
The power is not granted to the Federal Government under Art 1 Sec 8. Clause 5 does grant the power to 'Coin Money' but that is only the physical act of stamping the coins. It also grants the power to 'Regulate the Value thereof' which refers to the definition such as a 'dollar' is 371.25 grains of .999 pure silver.
All of the names of national currencies (francs, marks, pounds, etc.) are a form of weights and measures. The contain 3 elements, (1) substance, (2) amount, (3) purity. In other words, silver, 1 gram, .999 pure.
There is a good discussion by Robert Landis, a Harvard Law School graduate,
here around the middle of the video.