Elemental keys (エレメンタル·スピリット, Erementaru no Kei) are Magical Artifacts that are used for summoning the Elemental Spirits in a similar manner to Silver/Gold keys are used to summon Celestial Spirits. The Elemental Spirits are a race inhabiting the Celestial Spirit World and are considered to be most commonly found keys if you know where to look.
While the Silver and Gold Keys call the Celestial Spirits who are representative of the stars in space, the Elemental key's represent the elements. They live in a special part of the Celestial Spirit World. While they are a separate race, they have agreed to (for the most part) abide by the Spirit King's rules. Their keys can be found in their natural environment. They usually take a younglike appearance, most commonly a boy or a girl, but in some cases they take the appearance of an animal commonly found in their respective enviroment.
Each type of the Elemental Spirits have their leader, the strongest one among them. It is said that their keys are the hardest to find and that their strenght rivals the one of the Golden Keys themselves. They often refuse to make a contract with the key finder, but those kind-hearted usually make an exception and form a life-lasting bond, as seen with Arthur Moshiyoto and Voda, the leader of Water Spirits.
As stated before, there are numerous spirits and their respective keys and their strenght varies from almost useless to strong enough to conquer a Zodiac Spirit. Because of the difference in power level between two spirits of the same kind, Elemental Spirits are commonly classified through an alphabetical order, varying from E-Rank, D-Rank, C-Rank, B-Rank, A-Rank and S-Rank with E being the weakest and S being the strongest. The spirits are sorted into their catrgories based on their power level and endurance, as well the ability to remain summoned for a longer period of time. Ironically, C-Rank Spirits are the most common elemental spirits, the reason remains unknown.
E-Rank spirits aren't really combat-oriented, but their lack of fighting skills can be replaced with their cheerfulness and optimism. Their magical abilities are pretty weak, allowing them only to use the basics of their respective nature. As the rank grows, so does their power. C-Rank spirits are capable of putting up a proper fight with a silver-type key, while the S-Class ones' power is equivalent to those of a Zodiac Spirit.
Elemental Keys are magical artefacts that can be used by Celestial Spirit Mages or Cursed Spirit Mages in order to summon an elemental spirit.
To summon an Elemental spirit, you need to put the key through a Magic circle of the same element (Fire Magic Circle to summon Fire Spirit, Water Magic Circle to summon Water spirit etc.). However, if the bond between the spirit and the owner is strong enough, the spirit can be summoned without the presence of the Magic Circle. The Eternano used while summoning an Elemental spirit is equal to eternano used while summoning a Silver spirit.