The bowels of the Earth are full of rare gems. They will be useful on your journey.
They are used for buying more free spins in different slots, for example, Mystery of Egypt. They also help you open the chest, and unlock the Hourly Bonus game 'Spin', ahead of time.
Earn more gems from the Treasury, and the Daily Bonus. As an alternative, you can purchase a package of gems in the coins/gems buy menu.