There are two currencies in the game; Crystal and Amethyst. Crystals can be earned in the main quests and the Domain of Trials modes by defeating enemies. The premium currency of the game is Amethyst and it can be earned by completing the party quest or by watching the video advertisement when the option to do so appears once every six hours. It is advised that you do this as it allows you to get 50 Amethyst with minimal effort. You can also get 100 Amethyst free when you connect your Google account to the game which you should consider doing anyway as it will save your progress in the game and the Amethyst can be used to buy boxes for companions.
Amethysts can also be used to upgrade your weapons. This needs to be done regularly if you want to remain competitive in this game. A good way to invest your Amethysts is to wait until you have 50 Amethysts before upgrading your first weapon, which will give you two times EXP than usual upgrading. If you are lacking of Amethysts, you can sell your unwanted treasures to buy the upgrade package.