A personal headworld wiki
Crystali are creatures that are very closely linked to Oerba's core which constantly radiates energy. This energy is continually absorbed by them and radiated out through the mist in their tail. Crystali without the clusters allowing energy to flow through them will not have access to this power.
As the energy goes through their bodies, it is converted to their element. Being able to take in more energy at will to expell it as a 'spell' is a skill that one can either be born naturally good at, or that one can train and improve to varying degrees. Usually the more crystals growing out of a Crystali's body, the more ability to manipulate magic they'll have.
All crystali are born with an element. Elements can't be changed after birth and are usually genetically passed on to their offspring. In extremely rare cases, they can be born with 2 elements. This phenomenon tends to stay within royal bloodlines and is INCREDIBLY rare.
If a crystali is born with interrupted flow in any way, they still retain their element but they have no way of using it unless they connect with a source that isn't Oerba's core.
It's widely understood that the core of Oerba is a seemingly endless source of energy to be harnessed by its people. Knowing this, it's said that some savvy individuals have figured out how to create similar sources in weapons designed to amplify their magic. The creation of such things is considered highly immoral and their power can be compared to a nuclear weapon if the holder has proper control over it. Additionally, these sources need to be contained in Crystali crystals which would only work if harvested from a live donor.