The in game currencies are Silver, Gems & Gold
It is needed for most things you do like entering tournaments, merging cards with Conversion, replaying Scenarios or buying cards.
Gems are a bit more rare, you can find them in some Scenarios, there is a chance to win them in the Confrontation Power Roulette , and as a joining reward for the tournaments you will get 50 gems as well. They are also used as a reward for completing certain Achievements .
You mostly use them to buy cards and Booster Packs .
Gold is very rare, usually you have to buy it but occasionally it was added as an extra reward for Tournaments or completing 20 wins in the Arena . On a few occasions Confrontation gave away some gold as a gift for all players.
The best cards and Booster Packs are usually only obtainable with gold and with Evolution you also need gold to create the most powerfull cards. It is also needed to make Foil Star versions of Mythical and Legendary cards.