The Jade Gown is a mid to high-ranked mystic armor in Final Fantasy XII that boosts the wearer's Defense by 44 (Zodiac) or 33 (original), and Magick Power by 8. In the Zodiac versions, it also gives 53 MP.
In the updated Zodiac versions, the Jade Gown is sold for 8,100 gil in Balfonheim Port. It is a treasure in Feywood's Ice Field of Clearsight (75% chance to appear, 50% chance for gil, 50% chance the item treasure is the Jade Gown without the Diamond Armlet equipped), and in Necrohol of Nabudis's Cloister of Highborn (30% chance to appear, 30% chance for gil, 50% chance the item treasure is the Jade Gown without the Diamond Armlet equipped). It is still rarely stolen from the Topstalk in Feywood.
Jade Gown is a mid to high-ranked mystic armor, giving 44 or 33 Defense depending on version, and 8 Magick Power, as well as 53 MP in the Zodiac versions. Mystic armor offer magick-related bonuses that affect all types of magickal actions, from casting black magick to damage an enemy, to casting white magick to heal the party. Magick Power is also a factor when determining damage dealt by katana, staves, and maces.
In the Zodiac versions, White Mage, Red Battlemage, Black Mage, and Bushi can equip it, and so the Jade Gown is only useful to buy if the player has chosen any of these License Boards for their party. In The Zodiac Age versions, after fighting Belias the player can choose two License Boards per party member; it is usually a good idea to pair jobs that use two different types of armor for versatility.
In the Zodiac versions, the Jade Gown is a treasure in the Necrohol of Nabudis, so the player who ventures there early could score it much earlier than it is available from the shop. This area is optional to visit and thus possibly too tough for the party to brave as soon as the route there opens (after the Tomb of Raithwall).
Jade is an ornamental rock. The term jade is applied to two different metamorphic rocks that are made up of different silicate minerals. Nephrite can be found in a creamy white form (known in China as "mutton fat" jade) as well as in a variety of greens, whereas jadeite shows more colour variations, including blue, lavender-mauve, pink, and emerald-green. Of the two, jadeite is rarer. Translucent emerald-green jadeite is the most prized variety. For the Final Fantasy XII armor piece, the Jade in the name likely refers to the garment's color.