The fusion skill.
Fusion is one of the first 3 (or 4) skills you get in your skills panel. Its maximum level cap is 45, and to fully upgrade it needs 1035 skill points, and would waste 7,245 mana.
Skill Attributes
Whenever you upgrade this skill, you get -1.5% mana cost for gems. Every three levels will give you -1% gem combining mana cost.