Not to be confused with Jewels, a crafting item
Gems are different from the ordinary currency in the game, which is Coins. Gems are the premium currency, purchased with real world money. Certain items in the Market: some Buildings, Decorations and Natural resources can only be purchased with Gems.
Gems can be used skip parts of Quests. They can also be used to speed up collection times. Unfortunately, it is too easy to hit this option by accident. The option is jokingly referred to as a "gem trap." It is best to plan your purchases before obtaining gems, and then buy them right away so that the gem traps don't deplete your balance.