Spell Visualization

Different schools of magic have different casting styles, different looks to their spells, but also their spell slots are 'visualized' or conceptualized differently.

E.g. a wizard might see his spell slots as layers of pure and academic energy. These are sometimes called the supernal realms. There might be, for example, an endless expanse of dusty glass that represents cantrips, a large plain of silvery crystal for 1st level spells, and a jagged mountainside of ruby for 2nd level spells. Another mage might picture pools of liquid energy, or beautiful runes, or leaves hanging on a magnificent golden tree, etc.