The Diamond Armor is a mid to high ranked heavy armor in Final Fantasy XII that boosts the wearer's Defense by 40 (Zodiac) or 39 (original), Strength by 7 and Vitality by 5. It can thus be a good helm for those who use weapons that take the wielder's Vitality stat into account: axes, hammers and hand-bombs. However, Strength has a bigger impact on the damage dealt and so any armor piece that offers more Strength than the Diamond Armor will likely be better. The Foebreaker in the Zodiac versions equips the weapon types affected by the Vitality bonus, and so Diamond Armor might be especially good for them.
It rarely drops from the unarmed Baknamy in the Salikawood. It is also available through the Hunt Club sidequest on the Phon Coast, by giving Blok five Rare Game trophies and then buying the Diamond Armor from the Shifty-Eyed Merchant's store nearby for 6,300 gil.
In the updated Zodiac versions, the Diamond Armor is sold for 6,600 gil in Mosphoran Highwaste after defeating Judge Bergan, and in Phon Coast. It is found as a treasure in Mosphoran Highwaste's Trail of Sky-flung Stone (75% chance to appear, 40% chance for gil, 50% chance the item treasure is the Diamond Armor without the Diamond Armlet equipped), and Tchita Uplands' The Highlands (30% chance to appear, 70% chance for gil, 50% chance the item treasure is the Diamond Armor without the Diamond Armlet equipped), as well as in Trial Mode Stage 96 with the Diamond Armlet equipped. It still rarely drops from the unarmed Baknamy in the Salikawood.
Diamond Armor is a mid to high level armor giving 40 or 39 Defense depending on version, 7 Strength and 5 Vitality. The strength bonuses make heavy armor ideal for frontline fighters, and the Diamond Armor is a good armor to equip over any inferior equipment the player may have on anyone who attacks physically, although a gun-wielder does not benefit from the strength-boost. Vitality affects the duration of statuses; ailments are shortened while buffs are lengthened. Higher Vitality decreases the success rate of landing negative statuses, and Vitality is also in a small way accounted for in the damage calculation for axes, hammers, and hand-bombs.
In the Zodiac versions, Uhlan, Knight, Time Battlemage, and Foebreaker can equip it, and so it is only useful to buy if the player has chosen these License Boards for the party. In The Zodiac Age versions, after fighting Belias the player can choose two License Boards per party member and so more characters can benefit from heavy armor, as it is usually a good idea to match jobs that use different types of armor to bolster light armor jobs' strength, and to give more durability to spellcasters.