Cannot merge gem in my bag

I can merge all gem in my stash but not the one in my bag and im new to that feature, but i couldnt find easily if there was a way to withdraw on kind of gem or drop one kind in my stash only. Would be nice to be able to merge even the ones in my bags. For now my only solution is drop it all in the stash merge then take out what i need which is fastidious. The gems that are in my bag just dont appear in the list to merge. Maybe im just too new to the feature and ill figure it out but i didnt at first look.