There can be a lot of active events and offers in the game. That's why not all icons are visible on the right side of the screen. To find collected crystals in the Piggy Bank you need to scroll the list on the right side of the screen. You can use the button at the end of the list or just swipe up if you play on a mobile device.

Press it until the Piggy Bank icon appears, then press on its icon to view and get crystals at a discounted price.