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definitely not. I am level 435 and have played for over a year. The gems for Missions have actually decreased. Rovio has screwed with the game economics so many times I couldn’t put a number on it. At this point, Missions that have a question mark that generate a gem reward give 1 to 12 gems regardless of hard, medium or easy Mission and regardless of length (4, 8, 12 hour). I have gotten 1 gem on a 12 hour hard mission with three level 15 TFs. In fact on that Mission, I got two gem rewards, and … wait for it… a 1 gem and a 3 gem! ohhh my, so generous.
I can say the averages have not increased but rather decreased because at this point I am not playing nodes, just running missions. Used to be able to collect about 120-150 gems per day with Missions and gambling in the lab. Since the halloween update, that has dropped to more like 70-80 per day.