To add on to my comment: Having to combine medium gems from the side, but large gems from above, makes it IMPOSSIBLE to rig them up with the way the game is set up. The biggest problem is that you can't choose which gems combine.
Horribly impossible concept. Between the inability to move large gems (unless you have other ones of them) and the inability to combine things from the side, it is LITERALLY impossible to win most of the levels. 1/5 for this failure.