Needs different enemy types, like a Tank[3 times health, but half as fast], a speed [2times faster, 1/2 health], or maybe a Boss[ every 5 or 10 waves, 25times heath, but 1/5 speed]
You should have a "Are you sure"? button after a gem purchase, because I accidently bought one of those "speed up crafting things. Also, There should be ways to spend amounts of gems less then 5 besides speeding up crafting.