Gems and crystals are stored locally on your device.

1) Collect a lot of them.
2) Backup your game data using backup software like Helium or adb backup.
3) To be safe, make sure you choose the cloud backup in the game. This does not backup your crystals and gems.
4) Spend away all your crystals and gems.
5) Backup to the cloud again in game.
6) Restore your data from number two.
7) Repeat steps four to six as long as you want.