In one of my worlds, magic is operated by manipulating 'mana' that flows invisibly round the world, a bit like rivers. In places it wells up to the surface, and in others sinks down. Where there is water, it goes down deep, Some places the mana flows join, and at these 'nodes' as they are called, magic is stronger. There are 5 known schools of magic, although there is a theory that a sixth exists. They are, Matter, Energy, Spirit, Mind and Time. In another world in a yet unpublished book, magic can only be used in ggems imbued with it by a magician.