A fun little game with potential!
I found that at times it was both hard to turn (when moving) and too easy to turn (when not). I think there needs to be some active way for the player to get rid of the ships, like dropping bombs -- you could do shooting, but that's not that unique. After a while of evading ships, was hard to actually find where the crystals were. Play area for these kind of Asteroid like games tend to wrap around for a reason, so that you don't get lost. You might want to add some kind of indicator that follows the edges of the screen to tell the player what direction to head to the nearest crystal.