Upgrade Jewels are jewels used to upgrade a Character.
Each character can be upgraded at level 20, 50, 80, 100, 120, 130, 140, 150, 160. Upgrading the character increases the character's stats and unlocks some abilities.
The first 2 upgrades, levels 20 and 50, use Crystals of the same type as the Jewels that are used for the remaining upgrades.
For example, for Legion Mercenaries, upgrades unlock and increase the mercenary's skills. For Main Characters, the level 100 upgrade unlocks an additional skill.
For more information, see Upgrade and Main Character Upgrade.