Construct the Crystal Vault to store, activate, and craft Crystals. There are two action tabs in the Crystal Vault: Activation and Crafting.
The Activation tab contains a Cache, wherein your Crystal Coffers and Crystals are stored, and an Active Crystals panel, which allows you to activate your Crystals and therefore improve certain statistics of your Units.
You start with one open slot in the Active Crystals panel; in order to open the other two you will need a Magic Key - you can find one at the Black Market by tapping on the Misc. tab. You can also obtain Magic keys on Days 15 and 32 of the Daily Loyalty Roll. In case you need to replace Crystals in the Active Crystals panel you can salvage a Crystal and put another one into the slot.
Important: while you can salvage Level 1-4 Crystals for free, you will need a Goldsmith to salvage Crystals of Level 5 and above. If you attempt to salvage such a Crystal without the help of a Goldsmith, it shall be destroyed. You can hire a Goldsmith at the Black Market.
The Crafting tab enables you to upgrade your Crystals. This can be done as follows: select two Crystals of the same type and Level by tapping on the corresponding icon and then tap on the green Craft button. You will obtain a Crystal of the same type, but of a higher Level. For example, two Level 1 Crystals will be crafted into a Level 2 Crystal. Remember: in order to craft high level Crystals, you must first upgrade your Crystal Vault. The maximum Level of your Crystals is 12.