Air Diamond

A powerful diamond, imbued by a group of great wizards, with the power to summon a elder tempest from the Elemental Plane of Air.  They created this and sealed it away, along with the other 3 elemental crystals, in case there was ever a dire need to summon the Elder Elementals to save the world.  The power that resides in these is so much, that once they are used to summon their respective Elder Elemental, they shatter and are not usable again.  Whoever summons the Elder Elemental is in complete control of it until the creature is dismissed by the summoner.