Diamonds or jewels are the currency of the game. Almost everything can be accomplished if you are willing to use your diamonds for that purpose.
You can obtain diamonds in several ways. The accomplishments in the commander’s area pay in diamonds. Rewards for winning activities often include diamonds. The shooting gallery can award diamonds.
Diamonds can be mined in the territory outside the city. Or, you can spend real world money and buy diamonds.
It is not easy to accumulate a supply of the jewels. Mining them is slow, with a fixed rate of three per hour. There are ways to speed that up some but it is still hours to obtain less than 100 gems.
The shooting gallery can provide diamonds, under targets, at random. These are usually 10 or 20 at a time.
Being a part of winning an activity is often the best way to get diamonds. Whether it is the Single Player Arms Race, Zombie Siege or a Cross State Battle, winning results in rewards that often include diamonds.