Crystalization

You can crystallize unwanted in-game cards to receive crystals, which you can use to upgrade and fuse your other cards. The amount of crystals you get depends on the card’s rarity and its level of enhancement.

Please note that crystallizing your in-game cards destroy them in the process. On the other hand, when crystallizing a blockchain card, it doesn't disappear entirely; instead, it gets downgraded to its minimum gold level.