During the game you will come across random items that drop from enemies in the form of 'Gems'. You will then be given a tutorial on how the 'Gem Slot' system works as the game walks you through the first time. Afterwards you will need to unlock 3
slots which will require 1000 Silver
To unlock them, follow the below guide.
-> Inventory -> Hover Over The Gear That Doesn't Have a Gem Slot Unlocked ->
(Or A) and you will unlock the gear slot.