Not really enough information here. Despite most people assuming magic items are immune to most damage (to be fair, there’s a table of quirks to give magic items that includes this and all abilities that explicitly target objects like rust monster etc specify non magical) RAW they just have resistance to all damage unless they’re artefacts. If the archduke has the magic item he can just destroy it by smashing it to pieces or melting it down. If he doesn’t have it, he could send people to find and take it.

In combat, it’s assumed you can’t target items for the same reason you can’t blind someone by aiming for the eyes. Don’t start doing it, it ruins the game, your players will rightfully try to use the same strategies.