Once in a while, you may receive special offers to buy Crystals cheaper. You can purchase them but it is completely optional, playing without Crystals is just as fun and affordable because the game is free!

But if you want to speed up some processes and for this, you need event currency, you can buy Crystals anytime you're ready by pressing the plus button on the Crystal bar in the top right corner.

If you're not planning to buy Crystals at all, that's great too! Purchasing Crystals doesn't affect your in-game rewards and passing the story.