In Clash of Clans the way to upgrade, build troops, and do everything is by using Gold, Elixir, or Dark Elixir. However, the way the game developers makes money of their millions and millions of real dollars is from Gems.
Gems can be used in place of gold, dark elixir, or also used to speed up a building or X-Box that's being built or upgraded. Gems cost real money, and are the in-app purchase inside the game. And while the developers do give out Gem Boxes here and there, we're hoping for a Gem Mine, similar to the gold and elixir mines.
Don't expect this to ever come, but it's one thing everyone wants. Even if we received just one or two gems a day, would be plenty.