How do i mount the gems onto equipment please?

I don't know if its just a graphical bug or if the purple/bluish daggers get replaced by King's Claw, but it always seems to replace it or show up. I am not even sure you can get the purple/bluish weapon since they always turn into the King's Claw even though in theory I can get something stronger by now: https://steamcommunity.com/sharedfiles/filedetails/?id=2621983389...

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Hello, I would like to know how it is possible to restart the game once I have finished it?