An unopened Treasure Chest
Treasure chests contain various treasures and are 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 (or near) 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, by "trading" the appropriate key, 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 are only obtained if you do not already have it). Quest items take first priority when receiving items, with maps taking second priority, and the other items last.
Chests cannot be opened while invisible or weakened, and opening a chest will result in losing effects of Sneak and Deodorize, possibly in aggro from surrounding monsters, so special caution should be used when opening one.
Since the December 2008 update, treasure chests now repop approximately 5 minutes after being opened; however, if you (or anyone in your party) recently opened one and did not receive either a map or key item for a quest, you will be unable to open another for 1 hour 
. If you try to open another chest before this cool-down period expires, you get following message: "The chest was but an illusion"; the chest will despawn, but you keep the key.
roughly 3 minutes and 15 seconds respawn time on the chest if it was an illusion, will continue to be an illusion until it has reached the 1 hour time since it was last succesfully opened.
the time while the chest is not present (the 3 minutes between an illusion and it repopping) doesnt seem to be calculated towards the 1 hour cooldown have spent over 1 hour 30 mins trying to open a chest after roughly 9 illusions. so after every illusion add about 3-4 minutes onto the time before u can open it again.
See also
- Treasure Coffer
- Treasure Chest/Coffer Guide for information on specific chests
- Picking your Coffers and Chests for Thief-specific information