Two key items, Small Jewels, are found and made use of in Sol Sanctum. Described as "A beautiful eye-shaped jewel", each one is placed into a different minotaur-esque head carved into a wall. Doing so opens a hidden door elsewhere in the same room that leads further into the Sanctum, making it normally required to obtain and use these items to progress through the beginning of Golden Sun. Using the Retreat glitch, however, one or both of the locations for which the Small Jewels would be required can be skipped, meaning it is not necessary to obtain the items, and if they are obtained, they can be kept throughout the game.
As an Important Item, a Small Jewel can neither be bought, dropped or sold. None of this item appear in Golden Sun: The Lost Age under normal conditions, but it remains in the game code, and therefore can be obtained if transferred from the first game via Password data transfer. However, there is no logical reason to keep or transfer the items, as they have no use outside of Sol Sanctum and would take up permanent slots in the party inventory without any way to remove them in-game.