Diamond Bakugan are extremely elusive treasures that will only be found by the luckiest brawlers. Make sure to snatch them up if you see them, because once Diamond Bakugan are gone, they're GONE. Normal Bakugan will continue to be restocked after they sell out, but Diamond Bakugan never will.
When it comes to the actual game, here's how they work: Diamond Bakugan come with a rare Diamond Evolution card rather than a Character Card. All Diamond evolution cards can be pulled from TCG booster packs, but they're pretty rare. Diamond Bakugan must be played with the matching Character Card of their original Faction. For example, Diamond Dragonoid MUST be played with the Dragonoid Character Card. It cannot be played with other ones like Ventus Dragonoid or Dragonoid Ultra. You'll know it's the right Character Card if it doesn't have a Faction in the name; Ventus Dragonoid is the wrong character card because it says Ventus, but Dragonoid is the right card. It doesn't need to say Pyrus because the original Dragonoid is from the Pyrus Faction.
So, how do you evolve your Bakugan into a Diamond Bakugan? It's simple: Play the Diamond Bakugan card on top of the original Character Card like a normal evolution. Bakugan can only evolve into a Diamond Evolution from their original, unevolved Character Cards, and Diamond Bakugan can't evolve into any non-diamond evolutions. Choose your Evolutions wisely!