What color of gem is best to use in an amplifier

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.

The numbers refer to the maximum score multiplier you have obtained on that field. The white lines are simply a visual indication of how high your multipliers are - one line for every 5x multiplier, up to a maximum of 4 lines (20x). The shaded fields are fields with challenge amulets.