Answer:
small land in water.
I am a precious stone as clear as diamond. Seek me out while the sun's near the horizon. Though you can walk on water with my power try to keep me and I'll vanish in an hour.
I am the ultimate killing machine used not once but TWICE I can be made by humans.
What gets whiter the dirtier it gets?
I welcome the day with a show of light. I steathily came here in the night. I bathe the earthy stuff at dawn but by noon, I'm gone. What am I?
It flows out of the soil, It burns you if it boils, And holds us in its coils, More valuable than gold, As black as it is old.