What is the reward for breaking a tomb open in Gemcraft Labyrinth?
Firstly you get some extra monsters to kill. Also as JonathanJ mentioned you get 10% in your summon multiplier for every tomb you open. Note that all tombs are not the same. There are tombs with higher hit points and armor and the monsters they spawn are tougher to beat (Higher hit points and armor).
Generally it is good to avoid opening the tombs the first time you play a map. You will have the opportunity to come back and replay the map with higher multiplier settings when you reach a higher level.