Answer:
Three lives have I. Gentle enough to soothe the skin. Light enough to caress the sky. Hard enough to crack rocks.
Neither bought nor sold but more valuable than gold. It is built but not by hand. What is it?
In your fire you hear me scream, creaking and whining, yet I am dead before you lay me in your hearth.
My first half is normally made of stone, my second is crazy, and I taste sweet when complete.
A dragon's tooth in a mortal's hand, I kill, I maim, I divide the land.
I'm the source of all emotion but I'm caged in a white prison.