Psy Crystal

A Psy Crystal is a consumable item in the Golden Sun series. Like most consumable items, each party member can hold up to 30 Psy crystal in a single inventory slot. When used, a Psy crystal completely restores a single party member's Psynergy Points. Psy crystals can be bought for 1500 coins and sold for 1125 coins. Also, Psy crystals are considered Artifacts, meaning they can be re-purchased if sold or dropped.

Analysis
Aside from Psynergy Stones or spending a night at an inn, neither of which can be used during battle, Psy crystals are the most effective way to recover lots of Psynergy Points at once. Granted, there are plenty of other ways to restore PP: Djinn like Ether can be used indefinitely, while some armor, such as the Mythril Circlet or Mysterious Robe, gradually restore PP during battle. Also worth noting is that Adepts automatically recover PP as they walk through dungeons or across the world map. However, none of these methods work nearly as quickly as Psy crystals.

Unlike Hit Points, the consequences for running out of Psynergy Points are fairly mild. A character without any PP can still perform physical attacks, use items, unleash Djinn, and pretty much any other battle command except casting Psynergy. Also, PP can be restored through normal means even if a character has zero Psynergy Points remaining. These lax punishments, coupled with the rarity of Psy crystals, should compel players to conserve Psy crystals for situations where a character is almost out of PP. Also, if a player has very few Psy crystals available, they should be saved for when that character has a Psynergy spell that would provide a strategic advantage.

Psy crystals are perhaps best reserved for boss battles. The most obvious reason is that boss battles take longer than normal battles, putting a larger strain on a party's PP reserves. Another, less-obvious reason is that single-target Attack-based Psynergies like Odyssey and Liquifier are a simple and common strategy for defeating bosses faster. Unfortunately, such Psynergies are usually learned by classes with low PP totals, putting an even greater strain on those characters' PP pools. Should these characters run out of PP while a boss is still standing, a single Psy crystal may provide just the boost needed to win.

Prior to Golden Sun: Dark Dawn, Psy crystals were the only consumable item able to restore PP, making them vital items in late-game inventories for both Golden Sun and Golden Sun: The Lost Age. However, Dark Dawn introduces two new items (Khiren water and ginseng), both of which are considered "common items," meaning players can purchase a limitless supply of them. Also, the removal of certain minigames have made Psy crystals harder to come by. Although ginseng is easier to stockpile and may, as a result, see much more use, there is no denying that Psy crystals are the fastest way to replenish PP while outside city limits, especially during battles.

Trivia

 * Before Golden Sun: Dark Dawn, Psy crystals were the only consumable item that restored Psynergy Points. Even with the new PP-restoring items introduced in Dark Dawn, Psy crystals are the only ones considered Artifacts.