u/Due-Plate-1722

I can't improve my weapon... Is this due to a bug in my UI that hides the last item from me? 
However, my forge is level 3, I don't understand that.

I'm new to the game and I'm trying to understand something.
My weapon already has the lightning element (Voltaique).
How will the gems I add work?

If I add a lightning gem, what would that do?
If I add a fire gem otherwise, will it cancel the lightning?

Thank you for your help