These are coffers containing treasures that are found in some dungeons throughout Vana'diel. Coffers are typically in higher level areas and have better contents than Treasure Chests. In order to open a coffer, you either need the appropriate key, or need to be a Thief with a lock-picking tool. The keys to open the coffers are dropped from specific mobs, which are usually located in the same zone as the coffer.
When you open a coffer you will either randomly get one of the possible items, some gil, or a quest item if you have a pertaining quest active.
Coffers cannot be opened while invisible or weakened, and opening a chest will result in losing effects of Sneak and Deodorize, possibly in aggro range from surrounding monsters, so special caution should be used when opening one.
Treasure coffers repop approximately 5 minutes after being opened; however, if you are not openng the coffer for a key item or item for a quest (such as Artifact Armor), you may get following message: "The chest was but an illusion..."; the coffer will despawn, but you keep the key and don't lose sneak or deodorize status. When you receive this message, treasure coffer will repop within 5 minutes.