Besides gold, elixir and dark-elixir, there’s also an in-game currency that can help you with building, upgrading and gaining more raw materials, just to name a few. This currency is called Gems and you earn them by completing achievements, clearing obstacles, or by completing certain events. But by far the easiest way to obtain Gems is by making in-app purchases. You can do this by using Google Play or Apple Gift Cards, for example.