The wisdom of a wizard

Part of a dialog between pupil and teacher, before Hogwart's was a gleam in J.K. Rowling's eye:

"Sir, all these charms are much the same; knowing one, you know them all. And as soon as the spell-weaving ceases, the illusion vanishes. Now if I make a pebble into a diamond" -- and he did so with a word and a flick of his wrist -- "what must I do to make that diamond remain diamond? How is the changing-spell locked, and made to last?"

The Master Hand looked at the jewel that glittered on Ged's palm, bright as the prize of a dragon's hoard. The old Master murmured one word, 'Tolk,' and there lay the pebble, no jewel but a rough grey bit of rock. The Master took it and held it out on his own hand."This is a rock; tolk in the True Speech," he said, looking mildly up at Ged now. "A bit of the stone of which Roke Isle is made, a little bit of the dry land on which men live. It is itself. It is part of the world. By the Illusion-Change you can make it look like a diamond -- or a flower or a fly or an eye or a flame --" The rock flickered from shape to shape as he named them, and returned to rock. "But this is mere seeming. Illusion fools the beholder's senses; it makes him see and hear and feel that the thing is changed. But it does not change the thing. To change this rock into a jewel, you must change its true name. And to do that, my son, even to so small a scrap of the world, is to change the world. It can be done. Indeed it can be done. It is the art of the Master Changer, and you will learn it, when you are ready to learn it. But you must not change one thing, one pebble, one grain of sand, until you know what good and evil will follow on that act. The world is in balance, in Equilibrium. A wizard's power of Changing and of Summoning can shake the balance of the world. It is dangerous, that power. It is most perilous. It must follow knowledge, and serve need. To light a candle is to cast a shadow..."

(From A Wizard of Earthsea, by Ursula K. LeGuin)