I was wondering the same thing. When farming.. any good dd should have perma mp. It could be useful for players that don't have enough damage so they don't regenerate mp fast enough.
I only have a 5 slot brooch and I use red cats, ruby, opal, emerald and vital. I will use tanza instead of vital when I do Gludio, krofin instances or tougher areas.
If I had a 6th slot I would use either diamond or pearl.
In mass pvp you want to use amethyst, obsidian, cats eye, Tanza and ruby. If 6 slot brooch you may want to use red cats, if your damage is already high and most are 1 shot then use a jewel that will improve your defense. In my opinion topaz will be better in olympiad or 1 vs 1 than in mass pvp.
But then again... what do I know I'm 1 shot anyways no matter what i try