Would you really want to see the Polish government keeping items that belong to someone else.
of course, in case there is something valuable, not only as per international laws but also as per mere morality, Poland cannot keep belongings stolen from people
Several months have passed, but the golden train - to everyone's surprise - hasn't been driven out of this bloody tunnel yet. Thus, Poland cannot keep the belongings stolen from people. Thus, items that belong to someone else are safely kept under the earth all the time. As a result, no one is going to get richer, neither Poland nor anyone else.
The only consolation in these very sad circumstances is to read this sentence by InPolska again:
Personally I would not want to wear a ring or a necklace or whatever stolen from someone exterminated simply because Jewish (or anything else). Sorry, I have morals.
No necklaces, no rings, sorry. We have morals, but we have no items. Sorry.