Energy required to cast Spells, these are energies on the atmosphere In Mages, these would manifest as Seals on their limbs. Mages are able to channel their Mana through the palms of their hands and soles of their feet. This is the reason why Seals manifest on their limbs.
Mana is especially abundant in areas where corpses of Ancient Beings are found. The Mana emitted passively by these would result on the formation of Sapphires in the area.
Think of Mana as nuclear radiation. It can get toxic in high amounts. Creatures have varying tolerance to it.