What can gems be spent on?

Gems can be spent on a few different things:

• To unlock chests:
  
• As a substitute for gold anywhere gold is used:
  
  ^{This works nicely if you are missing a bit for an epic.}
• To purchase chests:
  
• And to purchase gold directly:
  

Well that's the main thing to spend them on - opening chests. Other than that you can go to the first tab "Shop" and buy Chests or Gold. But my recommendation is save up and only speed up chests when you need to, as buying chests is not really worth it. Maybe an occasional 1000G from gems to buy an epic (that costs 2000G) would be worth it though.