I have 2 unlocked trap gems. I used mass grind to grind all unlocked gems. Afterwards I went to that menu again and it told me I still had 2 gems to grind for 1000 granite. I accepted, it took my granite, but my gem count didn’t change. I was able to do this multiple times.
I’d suggest having trap not count towards the mass grind number even when unlocked. When you try the same with Itherian artifacts it gives you a granite cost but tells you “You do not have any non-Locked, unequipped artifacts” so you don’t waste your resources.