Combo in this game is pretty hard. You notice that each gem color have 3 levels right (they grow each time another gem next to them is popped-up). To do combo you basically need to have multiple level 3 gems next to each other. After the usual Chaining, first level 3 gem gonna popped-up and triggers the other level 3 gem next to it. Just like a domino, but you need to have multiple level 3s.