So one of the effects of Prestidigitation is that you can create a small, nonmagical trinket that lasts as long as the spell. In theory, could I use this to make a counterfeit coin with a successful Deception check?

Yes, but you would have to be very accurate so that people wouldn't be able to tell the difference.

And keep in mind that objects created by Prestidigitation only persist until the end of your next turn. So it will vanish completely in like... 6 to 10 seconds. So if you're tossing a coin into a pile it would probably get lost in the collection and you'd be fine, but if you hand a single coin to someone and they examine it even a little bit they'll see it vanish in their hands.

Quote from TransmorpherDDS >>

And keep in mind that objects created by Prestidigitation only persist until the end of your next turn. So it will vanish completely in like... 6 to 10 seconds. So if you're tossing a coin into a pile it would probably get lost in the collection and you'd be fine, but if you hand a single coin to someone and they examine it even a little bit they'll see it vanish in their hands.

   So technically, you could create an almond sized object with Prestigitation, swallow it in front of some important political figure, then see if that person would be willing to save you if you started choking on it.  If that person does nothing, no harm, no foul.  I'm kind of surprised more people don't use this spell to find dates.   

Quote from Nildecanter >>

So one of the effects of Prestidigitation is that you can create a small, nonmagical trinket that lasts as long as the spell. In theory, could I use this to make a counterfeit coin with a successful Deception check?

You could definitely use prestidigitation to create a small coin - that would certainly qualify as a small, nonmagical trinket.

Whether you could pass of this coin as a real one would depend on things not specified by the spell, which are thus up to the campaign setting and the DM of. Stuff I can think of that might be relevant to think about -

Does your character know what a coin looks like well enough to create a duplicate? If you are currently holding a coin you're copying it's obviously easier, if you've just heard those particular coins described but never seen one it's probably quite tough.

How intricate are the coins? Do they have a detailed portrait of a well-known ruler, with precisely specified patterns? Or are they just roughly circular bits of metal made in a crude crappy mold, and no two are ever alike anyway?

How on-guard is the person you're passing the coin off to? If you're paying a merchant a CP for a trinket that they sell dozens of per hour, probably not at all. If you're copying a platinum coin and trying to buy a sword with it, probably very.

You may need a Deception check. Or you might need a history check (to know what the coin SHOULD look like). Or no check at all if according to the DM it's obviously easy enough that you can do it, or obviously too hard for you so you can't.

I feel his may be a per DM, but say you make a lump of iron.  If you used a forge and melted it with other things, does it disappear since it's technically not that item any more?  And since this is out of combat, how do you measure time?

Each round of combat takes up 6 seconds (hence why 10 rounds equals a minute) regardless of how many combatants are actively involved. So objects created by prestidigitation, at best, remain for 12 seconds (two rounds worth of time). So if you can manage to smelt an iron bar in that period of time, it would probably have all kinds of weird effects, but I'd go ahead and say that it's impossible.

Maybe if you combined it with fabricate?

You would have to do it quickly, more of a slight of hand thing. Like back alley dealings where you can't count coin quickly. Show a stack of 10 platinum and quickly throw them into a bag where they disappear out of eyeshot. I've used it to fool an NPC that we destroyed a trinket we were sent to find after we found out he was evil. At the meetup he had assassins waiting, so I prestidigitated the trinket and quickly dropped it on the ground and cast a firebolt to where it landed so it was basically disappearing at the same time the bolt hit. A good deception check and he was never the wiser we still had the trinket. The spell is just amazing and can be used in so many different ways.

I struggle to imagine a time when one fake coin would be worth it given all that has to come together to pass it off as a real coin.

Honestly, it would be better if you were at least a level 2 Transmutation or Conjuration Wizard [Transmutation = turn your copper coins into silver coins || Conjuration = conjure up a small chest or bag of coins] or be a level 3 Sorcerer using Minor Illusion [Subtle Spell to eliminate the Verbal & Somatic Components for Minor Illusion]  on a chest that would be weighed to appear as if it was actually a small chest full of gold &/or platinum coins.

I have a changeling Thief Rogue/Hexblade Warlock that has Prestidigitation as a Cantrip. He makes up that he's saying a phrase in Elven, but he's really casting Prestidigitation to make a fake coin. 

Would the coin not last for the duration of the spell (1 hour?)?

No... The spell has a max duration of an hour for some effects. But creating small objects explicitly lasts only until your next turn. So those objects only last for around ten seconds

Oh. So those 10 seconds would be in real time right?

Quote from RevRawr >>

Oh. So those 10 seconds would be in real time right?

No, it’s ten seconds of in-game time. I don’t think there’s any duration in D&D that’s real time, though it could be an interesting fourth-wall-breaking kind of thing.

I mean if it's a social encounter.

A prestidigitated coin might last long enough to fool a vending machine... provided your setting has vending machines.

Quote from RevRawr >>

I mean if it's a social encounter.

 If it's a social encounter, then play it by ear. You make the fake coin, give it to someone, they look it over. A few sentences are exchanged and then when they look down again, the coin is gone.

also good for walking into a temple and tossing a coin into the box at the door if your character is a determined cheapskate.

at a market, you could have a lead coin and cast prestidigitation on it making it look gold...and keep casting it until the person receiving the coin puts it in his/her pocket or stops looking at it...that way there's no suddenly vanishing object/weight in their pocket (although there would be some weight difference when the spell ends).   Of course, you'd be standing within 10 feet, mumbling, and waiving your hands every other six seconds - so that might be weird.

Ooh. I see what you mean. And if they ask what happened to the coin, I could say they already put it away.

Since this rogue has a +15 in Deception and Persuasion that would work like a charm. Thanks buddy.