The Diamond Helm is a mid to high ranked heavy armor helm in Final Fantasy XII that boosts the wearer's magickal defense by 29 (Zodiac) or 25 (original), Strength by 5 and Vitality by 3. 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 helm that offers more Strength than the Diamond Helm will likely be better. The Foebreaker in the Zodiac versions equips the weapon types affected by the Vitality bonus, and so Diamond Helm might be especially good for them.
In the updated Zodiac versions, the Diamond Helm is sold for 7,000 gil in Mosphoran Highwaste after defeating Judge Bergan, and in Phon Coast. It is found as a treasure in Dalmasca Estersand's Broken Sands (80% chance to appear, 50% chance for gil, 50% chance the item treasure is the Diamond Helm without the Diamond Armlet equipped), and Tchita Uplands' Sundered Earth (30% chance to appear, 70% chance for gil, 50% chance the item treasure is the Diamond Helm without the Diamond Armlet equipped), as well as in Trial Mode Stage 96 with the Diamond Armlet equipped. It still rarely drops from the Grave Lord.
Diamond Helm is a mid to high level helm giving 29 or 25 Magick Resist depending on version, 5 Strength and 3 Vitality. The strength bonuses make heavy armor ideal for frontline fighters, and the Diamond Helm is a good helm 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.