What are Crystals for?
Crystals can be bought as an in-app purchase. You can buy Power-ups and Lives with Crystals. Crystals help you progress faster in the game.
How can I earn more Crystals?
Currently the only way to acquire Crystals is through buying them in the game.
Click the blue plus sign to add Crystals to your game.
Here is the Crystals purchase window for US players.