Legendary Gems provide powerful bonuses to Primary Equipment Slots. They can only be inserted into Legendary Sockets in the aforementioned equipment slots, and like normal gems they can be removed and replaced at any time. You can craft, upgrade and manipulate your gems at the Apprentice Jeweler.
Legendary Gems can be upgraded by combining multiple Legendary Gems of the same name.
List of Legendary Gems
There's a total of 23 different Legendary Gems in the Technical Alpha of Diablo Immortal.