Well that's an idea for a new vermin.
It is the shape and size of a gem. It lives in walls of gems. Dwarves will confuse it for an actual gem and take it to the fortress.
If one tries to use the gem in a craft it will run away and become misplaced. Though you can track it on the map .. and any dwarf with good eyesight can spot it and decide to bring it back to a stockpile or jewelers workshop.
So what does it do? It crawls to your food stockpile and consumes food. It is uncatchable by cats and thus kind of a menace.
Completely and utterly pointless addition to the game.
it must be done
Not a bad idea, but narrow. Imagine the possibilities:
A type of cavern-living snake that mimics a rusty sword, alongside actual rusty swords you can find, that uses dwarves to carry it to its goals. It's useable as a weapon, until it decides it's gone far enough and bites your dwarf's hand, leaving him weaponless against whatever he brought the weapon to fight.
Small, magical figurines that you scatter around your fortress. When a gremlin, monkey, kobold, or what-have-you comes to steal your fortress's wealth, it might grab a magical figurine and bolt, discovering its fatal mistake as the figurine, showing impossible strength, grabs its artifactnapper and throws it (propelling itself backwards signifigantly in the process); or stabs the thief with a tiny sword; or simply alerts you to the thief's presence.
Mimics.
Traders who sell cursed statues that animate and attack if you kill the traders from that civ, steal their goods, or otherwise give them a reason to shout a command word through the fortress to where you put your new *<<+gold statue of a dwarf+>>*.
A kind, hollow humanoid made of metal that your militia commander mistakes for a suit of fine iron armor and puts on, discovering his mistake after the creature decides it doesn't want to let go and leading to an incredible symbotic warrior, likely fighting on after the dwarven component dies. Bonus points if it can fire blasts of energy from its palms and/or fly.
Et cetera.