Metal Vessel

 Metal Vessels' (金属器, Kinzoku Ki'') are the objects (weapons, jewelry, accessories, etc.) that contain Djinn.

Acquiring
When one captures a dungeon and obtains a Djinn, their Djinn moves inside of an object. This object, so far in the series, is at complete random or something the Dungeon Capturer has on him/her at the time. Though this might just be because of the situation. Sinbad described that the Djinn will move to what ever one is most accustomed with.

Appearance
When a Djinn possess an object and creates a Metal Vessel, an 8 pointed star surrounded by a circle appears on the object. It usually glows when in combat and the power of that Djinn is focused at this point.

When a Metal Vessel is broken in or out of its transformed state it is possible to switch your Metal Vessel. Since the original reason a Djinn chose to exist inside that particular Metal Vessel is because it has been with the owner for a long time and the user is most accustomed to it to the point where it is like an extension of the user's body. The only way to switch is to choose and use a different weapon to the point where you are more used to that weapon than the original one. If you do this the Djinn should switch on it's own.

Household Vessel
Household Vessel are objects that hold a fragment of a Djinn's power. These powers are utilized by a Metal User's Household Members, which are people relatively close to their respective King Vesels.

Use
The ability to use a Djinn is used by a Metal Vessel. The Metal Vessel is what is used to perform a Djinn Equip and a Djinn Weapon Equip. When one uses a Djinn's Metal Vessel it drains Magoi and can leave the person very exhausted. Solomon's Metal Vessels use the White Rukh as food, while the Dark Metal Vessel use the Dark Rukh as food.

Magic
All Metal Vessels use the Magic from their Djinn to perform powerful magic. It is essentially the same as Magic users, except that instead of having a wide array of different magics, Metal Vessels focus and amplify on one type of magic. Since the wielders powers are amplified they only have to give a little Magoi.

When one has mastered their Djinn to be able to perform a full body Djinn weapon equip they are able to use an attack called Extreme Magic. Extreme Magic usually appears as the same symbol that appears on a Metal Vessel massive in size floating in the air. Dependent on the Djinn Metal Vessel different effects occur. Ren Kougyoku's extreme magic summons an enormous tidal wave that could crush a city.

It also possible for Metal Vessels of the same type of magic to combine their magic abilities as long as their Djinns are contracted to the same King Vessel. This is demonstrated in Hakuryuu's case.

Trivia

 * The eight-point star on a Metal Vessel is a reference to the Khatim Suleiman (خاتم سليمان), meaning Solomon's Ring. Islamic legends state that King Solomon captured Djinn with an eight-point star ring.
 * The Metel Vessels are based on the Divine Staff, a special wand that allows a Magician to receive a big amount of magoi from Ill Ilah.
 * It is told that Sheba and Ugo were the ones that invented Metal Vessels.