The 
Diamond Armlet is a reoccurring piece of armor that appears in many Final Fantasy games, but in Final Fantasy XII, the Diamond Armlet has the unique ability of improving the quality of items that you receive from treasure chests.
In the standard PlayStation 2 version of the game you can equip the Diamond Armlet on any character in the party to receive the Diamond Armlet’s beneficial effects. In the International Zodiac Job System and Zodiac Age versions of the game, this changes so that you now need to have it equipped on the party leader.
The Diamond Armlet can be purchased after you make your second visit to Balfonheim Port after completing the events at the Ancient City of Giruvegan and the Great Crystal.
It can be purchased from the Unlucky Merchant at South Bank Village in the Dalmasca Estersand for 20,000 gil. That being said, there are a few points prior to then where you can obtain a Diamond Armlet early, although the chances are still quite low.
Tchita Uplands
There is a spot in the Tchita Uplands where a treasure chest spawns that has a 50% chance of containing a Diamond Armlet. Travel to the Nameless Spring zone and save your game at the regular Save Crystal (note, there are two Save Crystals in this zone, one regular and one orange).
The treasure chest can be found in the bottom-right corner of the zone next to the Nameless Spring called the Garden of Life’s Circle. Use the screenshots below to help you find it:
Reset your game and reload it if the treasure chest does not appear or if it does not contain the Diamond Armlet.
Alternative Methods
You can also obtain a Diamond Armlet from a Mom Bomb in the Great Crystal although the chances are quite rare (3% chance without the 
Thief’s Cuffs).
You can obtain a Diamond Armlet in the Windtrace Dunes area of the Dalmasca Westersand. The treasure chest is located in the southeast section of the zone that can only be entered from the Shimmering Horizons. The chest has a 25% chance of appearing and can be obtained very early in the game.
You can also obtain one by initiating the first Trial (Stage 1) in Trial Mode. Find the treasure chest that contains the Diamond Armlet.
