Fusion is one of the first 3 (or 4) skills you get in your skills panel. Its maximum level cap is 45 but it an be inreased to 60, and to fully upgrade it needs 1035 skill points, and would waste 7,245 mana.
Whenever you upgrade this skill, you get -1.5% mana cost for gems. Every three levels will give you -1% gem combining mana cost. At level 45+15, you get -90% mana cost for gems and -60% gem combining mana cost, and at that level it would cost you 6 mana for a grade 1 gem and 48 mana for combining gems. So a grade 12 gem could cost you 110544.