There are different variants/levels of Archaic Essences. These are needed for installing Gems into your weapon at a Gem Forge. By exploring different Shroud Lairs across the map, found within the Shroud and marked as a ??? green area on the map before discovering. Within the shroud lair you can harvest Archaic Essence from the plants that have a glowing yellow orb. Different Gem Forges require the different variants of Archaic Essence.