@BlizzThe weapon is a very tricky subject, to be sure, especially because both weapons are already pretty irregular.
Anyway, I think I know who would be one of the worst gems for Mars to fuse with: Amber. Amber's love of freedom mixed with Mars' love for fighting to make them a very destructive personality, similar to Sugulite, and the mix of their weaknesses, Pride and oppression, would probably make so that they'd view anyone trying to command them to be someone trying to control and oppress them. Add on to that the combination of their two powers and you've got yourself one bad mammajamma of a fusion. The only problem is . . what kind of weapon to give him, as the flanged mace has already been taken.
I feel that a mix between Evenkite and Mars would have a similar effect, though it probably would be as bad.