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 these required to progress through the beginning of Golden Sun.
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 - but it, like all other important items, remain hidden and unused within the game code. More Small Jewels can be hacked into the party inventory in either game by an Action Replay device either directly or from the Debug Room, and they can be transferred between games via Password data transfer as well. In addition, by process of a glitch involving the Retreat Psynergy, either or both of the Small Jewels obtained in Sol Sanctum can be kept in the party without being used and then later transfered via password, without any Action Replay or third party device; of course, there is no logical reason to do this, considering they would take up permanent slots in the party inventory without any way to remove them in-game.