Correcting a misconception here, as per a conversation with Urianger in Shadowbringers.
Light is Umbral (the Aether of Inaction and Stasis. Thus why Alexander, a Primal of Holy Light, can stop Time.) Darkness is Astral (the Aether of Action and Motion).
Elements of motion (Wind, Fire and Lightning) "naturally" take Astral forms (though Umbral incarnations exist. A raging wildfire is Astral Fire, while the stiffling heat of the desert is Umbral Fire), and Elements of stasis (Water, Earth and Ice) "naturally" prefer Umbral forms (though again, Astral incarnations can exist. A rampaging blizzard is Astral Ice, while the all pervading cold of Coerthas or Garlemald are Umbral Ice).
This neatly ties into White and Black Magic using (or in the case of White, having once used) 2 elements of one type, and 1 of another. White Mages use Stone and Water which are umbral elements, and Wind which is astral. Black Mages meanwhile use Fire and Thunder which are Astral, and Ice which is Umbral.
This thread has a really nice and easy to understand writeup, but they're basically like the polarities of electric charge (Positive or Negative). Some things naturally possess a charge tending to one direction, but can be influenced in the other.
https://forum.square-enix.com/ffxiv/threads/392704
