Amethyst

Amethyst gems are characterized by their ability to attack only air enemies, large range, violet damage type and top of the line single-target damage. Special combinations they're part of tend to have area of effect.

All air waves, every fourth wave, are weak to Amethyst. Furthermore, land waves 7, 18, 34 and all above 40 are also weak to it, but Gold is the only gem that can deal both violet damage and hit land units.