[The Congress shall have Power . . . ] To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures; . . .
Article I Legislative Branch
Section 10 Powers Denied States
Clause 1 Proscribed Powers
No State shall enter into any Treaty, Alliance, or Confederation; grant Letters of Marque and Reprisal; coin Money; emit Bills of Credit; make any Thing but gold and silver Coin a Tender in Payment of Debts; pass any Bill of Attainder, ex post facto Law, or Law impairing the Obligation of Contracts, or grant any Title of Nobility.
And this seems relevant...