Treasure chests contain various treasures and are found in many dungeons throughout Vana'diel. In order to open a chest, you will need to use a treasure key, which are dropped from specific monsters in the same area that the chest is located or, occasionally, in an adjacent area. 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; which can only be obtained once. Quest items take first priority when receiving items, with maps taking next-highest priority, and all other items taking last priority.
Special caution should be used when opening chests: they cannot be opened while invisible or weakened, and successful opening of a chest will remove sneak and deodorize effects, possibly causing aggression from nearby monsters.
Since the December 2008 update, treasure chests will always reappear approximately five minutes after being opened. However, whenever a chest is opened to obtain gil or treasure items, all chests in the area will be reserved for artifact armor and key items for approximately one hour. Attempting to open a chest for other types of treasure during this period will cause the chest to disappear with the message "The chest was but an illusion." (The treasure key that was used will not be lost if this happens).