This animated show (possibly an anime) was about a guy with red hair. He has a device that can make him have armor and when he wants to, he can change the armor into something that looks like a gem (like a red ruby).
There were two other guys that have the same device but their device are blue and green, respectively. When they need a weapon, some of the armor disappears and becomes a weapon.
At one point they need to fight a monster. I think it was a 'gem monster'(?)
