I dont know if this is intended or a bug so this is either a suggestion or a bug report depending on what it is. You cannot swap gems if there are gems in both of the slots. If its a single gem slot item, you can't swap gems at all. If this isn't a bug and not being able to swap gems is intentional, I think we should have a way of removing / swapping gems, because not being able to get rid of a gem you don't want on your very rare weapon is very annoying.

Thanks for the report (and the included video - that really helps).

Being able to swap gems at all is not intentional - so you've discovered a bug.
I'll consider implementing working gem swapping or perhaps an item that allows you to scrub all gems from a weapon.

Hi!
Has there been any progress with gem scrubbing and is there a Discord for such quick questions?