If, against all odds, there are no other obvious uses for the astral gems, Acasric Record having been cast on all your provinces, and your global Arcane Nexus granting you an obscene amount of gems per turn.... you can always open the ether gates and let it the ether lords and warriors. Summoning a few clans will eat up gems really, really, fast.
And if you do not need that either, then you can start wishing.
And if you do not have anybody with the magical power to cast wish, then you can empower somebody to do so. That should take a few gems as well.
But, really, astral gems are always useful for
something.