Answer:
You use it between your head and your toes, the more it works the thinner it grows.
If you break me, I do not stop working. If you touch me, I may be snared. If you lose me, nothing will matter.
I pass before the sun, but make no shadow.
A dragons tooth in a mortals hand, I kill, I maim, I divide the land.
By Moon or by Sun, I shall be found. Yet I am undone, if there's no light around.
Iām grown from darkness but shine with a pale light. Very round I am and always a lady's delight.