Psynergy Stones are purple rocks that restore the Psynergy Points of the entire party. They can be found on the ground in many locations throughout both Golden Sun games, and they are pretty easy to spot. They are often placed at strategic locations, like the entrances to the interior of the Elemental Rocks, and near Lighthouse aeries. The small stones disappear after use, returning when players leave the area and come back, and the large stones, such as those in Vale and Air's Rock, remain permanently.
In Golden Sun, Isaac's party accidentally trigger the eruption of Mt. Aleph, causing the mountain to rain Psynergy Stones across Weyard. One of the most widespread effects of the stones turns wildlife into monsters that the party must fight, both as random encounters and as bosses, like Tret. The stones change geographical features, temporarily destroying the Kalay Bridge and transforming the Mogall Prarie into the Mogall Forest. But more importantly, many people around Weyard are exposed to the Psynergy Stones, and several of them gain Psynergy powers, the most notable cases being Feizhi and Eoleo.
In the sequel, this is expanded upon when the party visits the town of Garoh, where it is speculated that Psynergy Stones could be the cause of the inhabitants of that town becoming werewolves. There, Kraden and Maha assert that Psynergy Stones could be the source of power for all Adepts, along with the the remnants of Weyard's ancient past, such as Lemuria and the Elemental Lighthouses.
Golden Sun: Dark Dawn describes the Psynergy Stone as a Psynergy-rich mineral that forms over many years as elemental power slowly crystallizes.
A consumable item, the Psynergy Crystal, is a smaller version of Psynergy Stones, and can be used to refill an Adept's PP meter.