Diamond Armlet is an accessory in Final Fantasy XII that obtains superior/different treasures from coffers when the looter has it equipped. It has a more substantial role in the main game in the original PlayStation 2 version, but in the Zodiac version, it mainly helps with the Trial Mode treasures.
In the Zodiac versions, the armlet also nullifies Lightning damage to the wearer, perhaps as an allusion to the early series where diamond gear had this property. Wearing another piece of equipment with a "weakness" trait, the wearer remains immune. Thus, if equipping the Diamond Armlet in the Zodiac versions with the Venetian Shield, the wearer is still immune to Lightning.
In the original PlayStation 2 versions, the Diamond Armlet can be bought for 20,000 gil from Dalmasca Estersand after receiving the Treaty-Blade. It is also found in a treasure in Tchita Uplands' Garden of Life's Circle (near the middle of the southern border; 10% chance to appear, 50% chance for gil, 50% chance the treasure is a Diamond Armlet without a Diamond Armlet equipped ), rarely drops from the Mom Bomb, and is obtained from the Hunt Club sidequest on the Phon Coast (give 20 Rare Game trophies to Stok and then buy from Shifty-Eyed Merchant's store).
In the updated Zodiac versions, the Diamond Armlet is bought from the Phon Coast for 12,000 gil after the player obtains the Treaty-Blade. It is found in a treasure in Dalmasca Westersand's Windrace Dunes (25% chance to appear, but is always the Diamond Armlet), Zertinan Caverns' Sandfalls (always appears), and Subterra's Penumbra - North (70% chance to appear, 80% chance for gil, 50% chance the item treasure is a Diamond Armlet without a Diamond Armlet equipped), as well as in the Trial Mode Stage 1. The player can easily farm them by repeating the Trial Mode Stage 1 in The Zodiac Age.
Diamond Armlet is only effective when equipped on the party leader, or on Vaan when in towns. It affects the item content when the treasure (1) spawns (2) is not gil. Two items are available, usually splitting the chance in a 90–10 fashion (original) or 95–5 fashion (Zodiac), with the higher chance usually being a Knot of Rust. The two items can also be the same item. Cerobi Steppe is a notable area where all treasures can have a rare item when the Diamond Armlet is equipped, however, the chance to get the rare treasure from these chests is still very low.
In the original release, the Diamond Armlet increases the gil amount that chests give, but in the Zodiac versions, the gil amount is unchanged. One notable use for the Diamond Armlet in the Zodiac versions is the Trial Mode treasures, which can change quite drastically with it equipped. Whereas the player could ruin their chances at some treasures (such as some of the top tier equipment, like the Zodiac Spear) in the original version by equipping or not equipping the Diamond Armlet when looting a treasure, the Zodiac versions treasures usually always yield a certain item or ability no matter if the player has a Diamond Armlet equipped. The exception is the new invisible super equipment, which are mostly Easter eggs as they appear to be items originally used for testing the game and thus don't have models.
In the Zodiac versions, the Diamond Armlet also nullifies Lightning damage to the wearer, making it useful against Adrammelech, for example. It is worth procuring the armlet early for its Lightning-protection, though its effect on treasure loot is not really notable unless the player wants to collect Knots of Rust and Meteorites. It is now possible to farm the armlet from the first stage of Trial Mode, and then sell the armlets to make easy gil in the beginning of the game.
The player can equip it to a character they don't normally use and then switch to them when opening a Diamond Armlet chest to avoid changing equipment on the main party.