Early on in the game, all of your free gems will come from completing objectives, which usually are unlocked by doing something repetitively, such as winning five arena fights. Run down the list of objectives and complete the ones that are the easiest to complete. Fighting the story mode battles, in particular, will load you up early on, as each one completes a 50-gem achievement upon finishing the battle.

Once you do that and you have plenty of gems to play with, go to the operations area and spend gems in order to upgrade your operations. Load up until you have a significant percentage boost added to your gem earnings. The higher that you upgrade your operations, the better your chances of earning gems and the more that you'll earn.

Fighting in the arena is a great way to earn more gems, along with other bonuses. The more you fight and the higher than you rank up in the arena, the better the rewards that you earn. Rewards get doled out as each tournament session ends, which is weekly. Jump into the Elite ranks and fight to keep your place and you will earn free gems.

Even if you don't plan to play much for the day, log in once every day anyways to receive your daily reward. Many of the rewards are gems, and at the end of the calendar, you'll get a gem jackpot. Plus, complete your daily quests, as well. The standard quests don't always give you gems, but the daily ones will most frequently give you gems.

Plan for the future to get more overall gems. If you are going to make an in-app purchase, get a card or an elite card. These will not pay out all of their gems at the same time, but they will pay them out gradually and they will pay FAR more than will spending the equivalent amount of gems for the lump sum total. It's like a gem trust fund.