Gems can be equipped on your character giving you additional Power Rank and stats.
Gems can be levelled up using Gem Dust. Empowered Gems offer additional abilities.
You can have 2 Lesser Gems and 1 Empowered Gem per element, Water, Air, Fire and Cosmic.

Gem Boosters

Gem Boosters are used to increase Level Up chance and Double Level Up chance.

Lesser Gems

Lesser Gems fit in the Lesser Gem Slot for that specific element, they can increase your Power Rank and stats.

Empowered Gems

Empowered Gems act like Lesser Gems, but fit in the Empowered Gem slot and additionally have an effect.

Class Gems

Class Gems act like Empowered Gems but will only be for your class, it may alter how a specific ability works.