I found out an easy way to earn gems but is rather tedious in the game. (Most of you probably know it) But for those who don't... on Android Knights and Dragons (not sure how it works on apple) you can go to the shop in game and go to "Get Free Gems" then click "Video Offers" and you can just keep watching the same video and earn 1 Gem every time. It seems to be unlimited and I have earned about 100 gems so far which comes in handy. Just a tip. (Sorry if this has already been mentioned or you already knew this) Also someone can clarify if this works on apple.
Note: I KNOW this is not a "glitch" but rather a strategy.