Good enough a time waster especially if it's going to be on a tablet or phone. Two major flaws though;
Tapping on a spot will tap all of the gems on that point. This can lead to the player unintentionally hitting one of the forbidden gems. If this is meant to be played with fingers, that's gonna be a huge problem due to how inaccurate you could be.
The forbidden gems also become active during the "rolling" period, instead of as soon as the gem is displayed.
Overall it's okay. As for suggestions, try adding in more types of gems the higher your score goes!
Sorry but i think the concept is a little bit too weak, and the graphics doesn't help ..
Thanks for your feedback! I will improve the concept of the game, this was just a test of the core mechanics. The graphics were not the focus, they will be remade on the next versions.