I feel like the States may be able to issue notes, represented by cryptocurrency, backed by gold and silver.
The constitution allows the states to MAKE THEIR OWN LEGAL TENDER FROM GOLD AND SILVER CURRENCY in Article One, section 10:
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.
Now you might read that and think that cryptocurrency is a THING other than gold/silver but, consider this quote from this webpage:
Money and the Constitution
The Constitution contains only two sections dealing with monetary issues. Section 8 permits Congress to coin money and to regulate its value. Section 10 denies states the right to coin or to print their own money. The framers clearly intended a national monetary system based on coin and for the power to regulate that system to rest only with the federal government. The delegates at the Constitutional convention rejected a clause that would have given Congress the authority to issue paper money. They also rejected a measure that would have specifically denied that ability to the federal government (Hammond, 92). Although the Constitution does not state that the federal government has the power to print paper currency, the Supreme Court in McCulloch vs Maryland (1819) ruled unanimously that the Second Bank of the United States and the banknotes it issued on behalf of the federal government were Constitutional.
If the federal government only is permitted to issue money, coin or paper, then how could state banks issue money? State banks did not coin money, nor did they print any “official” national currency. However, state banks could print bills of credit in exchange for specie deposits.