Wheels are the currency used to play the Crystal Mine Game. One game costs one wheel to play. The player can store up to five wheels at a time, and once the wheels run out, Gems are needed to continue playing.
In the Windows version, one game costs four wheels to play. The player can store up to four wheels at a time.
How to obtain
Wheels are gradually replenished with the passage of time. Additional wheels may be gained by collecting from shops or from Treasure chests left by friends. In the latter case, the wheel will replace the Heart. Parasprites in Ponyville have a good chance (~50%) of dropping a single wheel. Also, certain Decorations have a small chance of producing a wheel when tapped. (Quick note: There are suspicions that the developers did some changes in the Hearth Warming updates, so that decors no longer drop wheels.)
In the Windows version, a wheel is awarded for logging in every six hours. Changelings in Canterlot also have a high chance (~75%) to drop wheels. Up to five wheels can drop from a single Changeling before it is defeated.