Treasure chests are objects containing various treasures, found in many dungeons throughout Vana'diel. In order to open a chest, you need a treasure key, which are dropped from specific monsters in the same zone that the chest is located. If you're a Thief, you also have the option to pick open the chest by using a lock-picking tool.
When a chest is opened, you will randomly receive either a single treasure item, which will be placed in your treasure pool; some gil, which will be split amongst your party; or a key item, which can only be obtained by the character that opens the chest.
Treasure items can include gems, armor, weapons, quest items, which will only be obtained if the quest is active, and maps. Quest items take first priority when receiving items, with maps taking next priority and the other items taking last priority.
Please note, that while you are weakened, you cannot open a Treasure Chest.