Coins are guaranteed by the country they come from.
Sorta. Not like it matters. At one time they were. No countries are putting out precious metals coins, which would be like dumping money from the countries vaults onto the populace.
They'll guarantee mints are accurate in fiscal counting of non-precious metals, turning it into non-important coinage to help the flow of transactions, as part of the delivery of currency into the money stream.
After that, its on its own. They won't guarantee shit. You could forge a nickel out of a perfect alloy, take it to the Washington Mint, and turn it in as suspected counterfeit nickel. Would they issue you a guaranteed good nickel? No. They'd keep the old one for evidence, and probably detain you for questioning.
You could come up with some really dastardly scheme to mint quarters. The implementation of which is really fucking expensive, to mint coins. Big machinery and whatnot. You'd have no way to deliver it into the moneystream. Its a big, cost-prohibitive procedure that the US Mint actually loses money on, bigtime.
Heres how cost-ineffective it is to counterfeit coinage: You could go out in your garage right now, cut off about 10,000 sections of 1 inch black iron pipe, write $10 on them... throw them in the back of a pickup truck. Deliver them to a bank. Don't even get questioned. Don't get busted. Get $10,000 for them. (not $100,000 - because counterfeit money is discounted in deliverable value to scale)
On your tenth visit, you get busted. You do some major fucking time.
Now... The actual counterfeiting would be like, way, way harder than that. And the likelihood of being busted, way better. I'll bet even the mafia discarded the idea of minting counterfeit coins. And they could get all of the shit delivered to an airplane hangar on credit, including the manpower.
The only way the country would guarantee coinage of precious metals, is if the issued value was so ridiculously high it would benefit the government. Like, if they were offering $1,000 silver rounds. You bring $1,000 cash dollars, the US mint would sell you a 1oz coin of .999 pure silver. And really, is anybody that moronic? The Treasury would back that shit up like a bad-ass motherfucker. Until China started minting exact replicas, turning $34 worth of silver into $1,000 immediately. Then we'd have to nuke China, and everything would go very badly downhill in a fireball tailspin. The obvious alternative is to horde metals and allow the retail market argue over the price.