Raises the strength of spells.
Description
Concentrate is a support ability in Final Fantasy IX for white mages that allows their White Magic and Phoenix summon restore more HP, or, conversely, deal more damage to the undead. Concentrate costs 10 Magic Stones to activate. Despite the in-game flavor text, the ability only affects spells that restore HP.
Obtained[]
Concentrate is learned from Robe of Lords and Rosetta Ring for variable amounts of AP; Dagger requires 80 AP where Eiko needs 90 AP. Wearing both equipment pieces at the same time masters the ability at double the pace.
Rosetta Ring is first found with Choco, by using a Dead Pepper at a mountain crack at eastern part of Lost Continent. 
The player could get it after they have the Blue Narciss if they advance in the Chocobo Hot and Cold quest until Dead Peppers become available. Rosetta Ring is also synthesized in Black Mage Village in the endgame, obtained for giving 10 Stellazzio coins to Queen Stella, and obtained from the Friendly Yan, by either defeating it in battle or by appeasing it with a Diamond.
Robe of Lords is the ultimate robe, first available as the top prize in Mene's Chocobo Hot and Cold rewards shop, but it would take excess grinding to purchase there. It can also be stolen from superbosses, synthesized in Hades's synthesis shop, and obtained as a reward for giving 12 Stellazzio coins to Queen Stella.
Mechanics[]
With Concentrate equipped, the character's Cure, Cura, and Curaga operate at 150% healing power, and Life, Full-Life, and the summon Phoenix's revive at twice as much HP. However, Full-Life already heals to full health, and so Concentrate is redundant for it. Against undead enemies, the curative spells would do more damage, but Life and Full-Life would have identical effect in instantly defeating them. With Concentrate, Phoenix revives the party at double the HP, but the damage against the enemy is unchanged.
Concentrate has no effect on reviving with items (Phoenix Down and Phoenix Pinion are also unaffected by Chemist, so there is no way to boost them) or on buffing spells like Might or Protect, ailment spells like Confuse, or Eiko's Holy. To empower summons, the player should use Boost. It has no effect on Dagger's inability to concentrate that happens as part of the story.
Use[]
Concentrate is useful on Dagger and Eiko for improving healing spells, Life, and Phoenix. It counteracts the halving effect from multi-targeting curative spells, and with it, Curaga easily tops the whole party up. It has less of an offensive use, as though Concentrate makes Cure, Cura, and Curaga deal more damage to undead enemies, Dagger and Eiko should just use Life to defeat them in one go. Undead enemies do not tend to appear in groups where multi-targeting curative spells on them would be beneficial. Concentrate also helps Eiko's auto-Phoenix, which may appear to rescue the player from a Game Over. With Concentrate, Phoenix can restore the party to full health from KO.
Though the ability is somewhat expensive to equip, it is worth using.