Answer:
I am the greatest teacher out of them all, but alas in the end I kill my pupils.
A precious stone, as clear as diamond. Seek it out while the sun's near the horizon. Though you can walk on water with its power, try to keep it, and it'll vanish in an hour.
What has six faces and twenty-one eyes?
Take one out and scratch my head, I am now black but once was red.
What can burn the eyes, sting the mouth, yet be consumed?
What do you purposefully put lots of in and on your body, but run away from when you encounter it outside?