Double-click to apply to an accessory, amulet, or ring with an unused upgrade slot.
— In-game description
Sunset Jewels can be salvaged from 4 items from each of the specific class Specialization Collections. Items 3, 5, 10 and 11.
For example, for Warriors:
(Learned from: Recipe: Sunset Amulet)
(Learned from: Recipe: Sunset Earring)
(Learned from: Recipe: Sunset Ring)