ダイヤの兜 / ダイアの兜 / ダイヤのかぶと [dia no kabuto] or 'diamond helm' in Japanese. Sometimes also as ダイヤヘルム / ダイアヘルム [dia helm].
Final Fantasy I
Stats: DEF +8, EVA +3
Equip: ?
Obtain: ?
Description: A helm made of diamond
Final Fantasy II
Stats: DEF +17, EVA -18%, MDEF +5%
Equip: all
Obtain: ?
Description: A diamond helm resistant to lightning
Final Fantasy III

Stats: DEF 21, MDEF 10, weight 1
Use: FRE, ONK, WAR, KNT, DRG, VIK
Buy: 20,000 gil (sell: ? gil)
Shop: Ancient Ruins, Invincible
Treasure: Sunken Cave, Temple of Time
Type: Head
Other: ダイヤの兜 in Japanese
Final Fantasy IV
Stats: DEF 9, MDEF 2, EVA 0, MEVA 2
Equip: Cecil, Kain
Buy: 10,000 gil (sell: 5,000 gil)
Shop: Tomra
Other: resists Thunder
Final Fantasy V
Stats: DEF 10, MDEF 2, weight +4
Equip: Knight, Dragoon, Samurai, Berserker, Mystic Knight, Mime, Gladiator
Buy: 7,000 gil (Sell: 3,500 gil)
Shop: Moore
Final Fantasy VI
Stats: DEF 27, MDEF 18
Equip: Celes, Cyan, Edgar, Setzer, Terra
Obtain: ?
Description: Helm studded with diamonds of exceptional hardness
Other: ダイアの兜 in Japanese
Final Fantasy IX
Stats: MDEF +20, SPR +1
Equip: Steiner, Freya, Beatrix
Abilities: Insomniac, Accuracy+
Buy: 3,000 gil (sell: 1,500 gil)
Shop: Daguerreo (equipment shop (disc 3)), Ipsen's Castle (Kumool's mogshop)
Description: Helm made of diamond
Type: Helm
Other: ダイヤヘルム in Japanese
Final Fantasy XII
Stats: MDEF 25, STR +7, VIT +3
Buy: 7,000 gil (sell: 3,500 gil)
Final Fantasy Tactics
Stats: HP +80
Equip (job): Knight, Dragoon, Samurai, Dark Knight, Onion Knight
Equip (character): Squire (Ramza, ch2+), Fell Knight, Holy Knight, Game Hunter, Templar, Sky Pirate, Sword Saint, Divine Knight, Ark Knight
Equip (enemy): level 22
Buy: 6,000 gil (sell: 3,000 gil)
Shop: (available after Alma joins in ch3) Lesalia, Castles
Description: A close helmet adorned with magickal jewels of extraordinary hardness.
Other: other jobs can equip with the ability Equip Heavy Armor
Crystal Chronicles
Stats: DEF +22
Equip: Yuke
Buy: Diamond Sallet -scroll, Diamond Ore, Orichalcum, 2,000 gil
Record Keeper

Diamond Helm (III)
Stats: DEF +36, MDEF +32, ATK +5, EVA 102 (lv1), ? (lv5), ? (lv10), ? (lv15), DEF +71, MDEF +63, ATK +9, EVA 102 (lv20)
Max Level: lv15 (initial), lv20 (first evolve), lv25 (second evolve)
Type: Helm, Rarity: ★★★★
Other: enables use of the Knight Shield II limit break when equipped by Knight

Diamond Helm (VI)
Stats: DEF +21, MDEF +19, ATK +3, EVA 99 (lv1), ? (lv5), ? (lv10), ? (lv15), DEF +51, MDEF +45, ATK +6, EVA 99 (lv20)
Max Level: lv10 (initial), lv15 (first evolving), lv20 (second evolving)
Type: Helm, Rarity: ★★★
Category: Equipment