Help coming up with a cursed coin puzzle

[Generic Villain]

Hey all. So I'm designing a dungeon where the PCs need to acquire 3 coins to unlock the final door. The silver coin was already snatched up by a rival, so will fight the PCs. The gold coin is attained by solving a cipher puzzle. I'm having trouble thinking of how to challenge the PCs with the bronze coin though.

I have a few thoughts.

-The coin is cursed. As long as it is carried, the holder suffers penalties. I may even have it heat up, forcing them to rush to deposit it before it burns them alive.
-The room is filled with bronze coins. Thousands of them. Only one is the real deal. The problem here is, how would the PCs find the right one? And even then, the only "threat" here is spending a lot of time sifting through coins.

Thanks for any suggestions.

[shroudb]

The bronze coin can only be carried by itself. As soon as either the gold, or the silver coin, are in close proximity, the coin teleports to the original room.

This happens without fanfare, so if it happened to be in a pouch, bag, etc, and the party fights the rival and gets the silver coin, when they later open the bag to grab the coins to put them on the door, suddenly the bronze one will be missing (back in the original room).

[Claxon]

My thing whenever someone mentions puzzles is caution, and having a conversation with your players before actually making any sort of puzzle (unless it's incredibly smack you in the face obvious).

Why? Personally I'm not a fan of puzzles in RPGs. They tend to rely on player knowledge instead of character knowledge, and when I've spent my whole day thinking and problem solving I'm not really in the mood for more of that after work when I'm trying to play an RPG. Not all players are like me, but I'm generally against puzzles of any sort, and so I just completely disengage when puzzles are presented unless they trivial and solved in seconds (an example that comes to mind is the Skyrim style door puzzle with three animals shapes in the room and you have to turn circle pattern thing on the door to match).

So before you put a lot of effort into a puzzle...make sure your players want to interact with it at all. If one player in a group of 4 is like me and walks away to snack or plays on their phone it's not too bad. But if only 1 person at the table is interested...well in my opinion that becomes a waste of everyone's game time.