This is more or less the idea I've got about elements as a whole (also, I've been, slowly but surely, writing a fan-fic on elements which features most of these concepts):
The Elements were initially just one. I would have called that Aether, if it wasn't already an Element. Let's call it Chaos. Chaos then divided into the twelve Elements we know through a Supernova (that fills thematically too, doesn't it?). Those Elements tried to get the power Chaos had, but, having their distinct traits, they all got something other don't, or are inclined to something (Fire just loves rushes, doesn't it?).
Before continuing, I would like to state that everything has an Element "seed" in itself. Objects with a Seed of Light are under control of Light, for example. This is exactly what made Chaos so strong: it could control EVERYTHING.
The objects which have the very essence of the Elemental Seeds are the Elemental Pillars which sustain the life forms of each Realm/Kingdom but are, at the same time, sustained by the Elementals of their own Element. Elementals are those who've got the most power in a specific Element, having even Dragons, who are the strongest life forms entirely made up of their element's quanta, under their control.
The Rare Weapons were forged by each Element's Elemental as soon as the Supernova divided the Chaos into the Elements using Shards to give them power. Shards are the most powerful remaints of the Chaos Empire: they are, indeed, the fragments of the Sacred Relic, which was the object which ruled in the Chaos, keeping things under control.
You might also have noticed how some Rare Weapons need Quanta from other Elements to get at their strongest: that's due to them being copies, adapted to fit their element, of the Sacred Weapons (Dirk, Long Sword, Gavel and Long Bow) and thus still having traits of Chaos which tried to uniform everything, getting both the strength of the Fire Seed with the calm of Water Seed and so on.
Although everything has been divided into Elements, though, something has kept the ability of use Quanta from multiple Elements: the Sacred Weapons. However, the scism didn't leave them untouched: having to deal with multiple kind of Quanta being broken up made their power fall greatly.
Other than those object who "survived" the scism, there are other creatures who, seeking unity again (or power, for the most selfish ones), created spell who can be casted by anyone or mastered the control of another kind of Quanta (Seraph is the only one I've already explained in my fanfic).
That's the most basic of the summaries I could put together, without going too much into details for spoilers regarding my fanfic.