Actually, Federally-recognized Native American tribes have a government-to-government relationship with the United States and ARE seen as being sovereign nations.
For all the good it does them ...
Like I stated, so is California and Texas. The Constitution grants only the Federal government the authority to "coin." There is no exemption to Native Americans outside of treaties. If they had a need for their own currency, they would already have it.
Ummmm, they do mint their own currency.
http://www.freelakotabank.com/lakota-currency/"Additionally, we authorize the private minting of our own private currency"
Their "currency" is as authentic as coins from the Franklin Mint. They are worth their weight in gold and silver, nothing more. I do encourage them to trade in precious metals instead of fiat, which the website states that is their goal.
"We encourage you to negotiate the best exchange rate from your national currency to Lakota or any other reputably-minted .999 fine silver." That means good luck getting what you paid for it.