A Jewel Colony is the pride and joy of an empire. I would recommend that a nation be limited to either ONE Jewel Colony OR ONE Joint Jewel Colony. You can't have two favorites.
But, in fairness, I would ask that a further rule be tacked onto the Joint Jewel Colony so that a participating nation can treat it as a regular colony and not gain the rare from it, essentially it would act just the same as a Joint Colony. For example, if the Socialist nations of the world wanted to create a Joint Jewel Colony with diamonds, and I thought diamonds to be utterly useless, I wouldn't mine it. But I'd still like to be represented on Socialist alliance's land. So I'd get the Rice, and no diamonds. If that's too complex, then just leave it as one OR the other.