I personally found red gems are your best friend. They seem to be by far the most effective if you have plenty of traps to slow the monsters down. Also, experiment with multi color gems on a lower level that is easy to beat. I recommend you only go dual color as tri color gems have too little chance of landing the special effects. So i suggest sticking to pure red gems, and combination of yellow green, green lime, or yellow lime. but pure is always your best bet if you can do it.
